[lhc/web/wiklou.git] / docs / hooks.txt

hooks.txt

This document describes how event hooks work in MediaWiki; how to add
hooks for an event; and how to run hooks for an event.

==Glossary==

event
     Something that happens with the wiki. For example: a user logs
     in. A wiki page is saved. A wiki page is deleted. Often there are
     two events associated with a single action: one before the code
     is run to make the event happen, and one after. Each event has a
     name, preferably in CamelCase. For example, 'UserLogin',
     'ArticleSave', 'ArticleSaveComplete', 'ArticleDelete'.

hook
     A clump of code and data that should be run when an event
     happens. This can be either a function and a chunk of data, or an
     object and a method.
     
hook function
     The function part of a hook.
     
==Rationale==

Hooks allow us to decouple optionally-run code from code that is run
for everyone. It allows MediaWiki hackers, third-party developers and
local administrators to define code that will be run at certain points
in the mainline code, and to modify the data run by that mainline
code. Hooks can keep mainline code simple, and make it easier to
write extensions. Hooks are a principled alternative to local patches.

Consider, for example, two options in MediaWiki. One reverses the
order of a title before displaying the article; the other converts the
title to all uppercase letters. Currently, in MediaWiki code, we
would handle this as follows (note: not real code, here):

        function showAnArticle($article) {
                global $wgReverseTitle, $wgCapitalizeTitle;

                if ($wgReverseTitle) {
                        wfReverseTitle($article);
                }

                if ($wgCapitalizeTitle) {
                        wfCapitalizeTitle($article);
                }

                # code to actually show the article goes here
        }

An extension writer, or a local admin, will often add custom code to
the function -- with or without a global variable. For example,
someone wanting email notification when an article is shown may add:

    function showAnArticle($article) {
        global $wgReverseTitle, $wgCapitalizeTitle;
        
        if ($wgReverseTitle) {
            wfReverseTitle($article);
        }
        
        if ($wgCapitalizeTitle) {
           wfCapitalizeTitle($article);
        }

        # code to actually show the article goes here
        
        if ($wgNotifyArticle) {
           wfNotifyArticleShow($article));
        }
    }

Using a hook-running strategy, we can avoid having all this
option-specific stuff in our mainline code. Using hooks, the function
becomes:

        function showAnArticle($article) {

                if (wfRunHooks('ArticleShow', array(&$article))) {

                        # code to actually show the article goes here

                        wfRunHooks('ArticleShowComplete', array(&$article));
                }
        }

We've cleaned up the code here by removing clumps of weird,
infrequently used code and moving them off somewhere else. It's much
easier for someone working with this code to see what's _really_ going
on, and make changes or fix bugs.

In addition, we can take all the code that deals with the little-used
title-reversing options (say) and put it in one place. Instead of
having little title-reversing if-blocks spread all over the codebase
in showAnArticle, deleteAnArticle, exportArticle, etc., we can
concentrate it all in an extension file:

        function reverseArticleTitle($article) {
                # ...
        }

        function reverseForExport($article) {
                # ...
        }

The setup function for the extension just has to add its hook
functions to the appropriate events:

        setupTitleReversingExtension() {
                global $wgHooks;

                $wgHooks['ArticleShow'][] = 'reverseArticleTitle';
                $wgHooks['ArticleDelete'][] = 'reverseArticleTitle';
                $wgHooks['ArticleExport'][] = 'reverseForExport';
        }

Having all this code related to the title-reversion option in one
place means that it's easier to read and understand; you don't have to
do a grep-find to see where the $wgReverseTitle variable is used, say.

If the code is well enough isolated, it can even be excluded when not
used -- making for some slight savings in memory and load-up
performance at runtime. Admins who want to have all the reversed
titles can add:

        require_once('extensions/ReverseTitle.php');

...to their LocalSettings.php file; those of us who don't want or need
it can just leave it out.

The extensions don't even have to be shipped with MediaWiki; they
could be provided by a third-party developer or written by the admin
him/herself.

==Writing hooks==

A hook is a chunk of code run at some particular event. It consists of:

  * a function with some optional accompanying data, or
  * an object with a method and some optional accompanying data.

Hooks are registered by adding them to the global $wgHooks array for a
given event. All the following are valid ways to define hooks:

        $wgHooks['EventName'][] = 'someFunction'; # function, no data
        $wgHooks['EventName'][] = array('someFunction', $someData);
        $wgHooks['EventName'][] = array('someFunction'); # weird, but OK

        $wgHooks['EventName'][] = $object; # object only
        $wgHooks['EventName'][] = array($object, 'someMethod');
        $wgHooks['EventName'][] = array($object, 'someMethod', $someData);
        $wgHooks['EventName'][] = array($object); # weird but OK

When an event occurs, the function (or object method) will be called
with the optional data provided as well as event-specific parameters.
The above examples would result in the following code being executed
when 'EventName' happened:

        # function, no data
        someFunction($param1, $param2)
        # function with data
        someFunction($someData, $param1, $param2)

        # object only
        $object->onEventName($param1, $param2)
        # object with method
        $object->someMethod($param1, $param2)
        # object with method and data
        $object->someMethod($someData, $param1, $param2)
      
Note that when an object is the hook, and there's no specified method,
the default method called is 'onEventName'. For different events this
would be different: 'onArticleSave', 'onUserLogin', etc.

The extra data is useful if we want to use the same function or object
for different purposes. For example:

        $wgHooks['ArticleSaveComplete'][] = array('ircNotify', 'TimStarling');
        $wgHooks['ArticleSaveComplete'][] = array('ircNotify', 'brion');

This code would result in ircNotify being run twice when an article is
saved: once for 'TimStarling', and once for 'brion'.

Hooks can return three possible values:

  * true: the hook has operated successfully
  * "some string": an error occurred; processing should
          stop and the error should be shown to the user
  * false: the hook has successfully done the work
           necessary and the calling function should skip
        
The last result would be for cases where the hook function replaces
the main functionality. For example, if you wanted to authenticate
users to a custom system (LDAP, another PHP program, whatever), you
could do:

        $wgHooks['UserLogin'][] = array('ldapLogin', $ldapServer);
  
        function ldapLogin($username, $password) {
                # log user into LDAP
                return false;
        }

Returning false makes less sense for events where the action is
complete, and will normally be ignored.

==Using hooks==

A calling function or method uses the wfRunHooks() function to run
the hooks related to a particular event, like so:

        class Article { 
                # ...
                function protect() {
                        global $wgUser;
                        if (wfRunHooks('ArticleProtect', array(&$this, &$wgUser))) {
                                # protect the article
                                wfRunHooks('ArticleProtectComplete', array(&$this, &$wgUser));
                        }
                }
        }
                                                    
wfRunHooks() returns true if the calling function should continue
processing (the hooks ran OK, or there are no hooks to run), or false
if it shouldn't (an error occurred, or one of the hooks handled the
action already). Checking the return value matters more for "before"
hooks than for "complete" hooks.

Note that hook parameters are passed in an array; this is a necessary
inconvenience to make it possible to pass reference values (that can
be changed) into the hook code. Also note that earlier versions of
wfRunHooks took a variable number of arguments; the array() calling
protocol came about after MediaWiki 1.4rc1.

==Events and parameters==

This is a list of known events and parameters; please add to it if
you're going to add events to the MediaWiki code.

'AbortAutoblock': Return false to cancel an autoblock.
$autoblockip: The IP going to be autoblocked.
$block: The block from which the autoblock is coming.

'AbortLogin': Return false to cancel account login.
$user: the User object being authenticated against
$password: the password being submitted, not yet checked for validity
&$retval: a LoginForm class constant to return from authenticateUserData();
          default is LoginForm::ABORTED. Note that the client may be using
          a machine API rather than the HTML user interface.

'AbortMove': allows to abort moving an article (title)
$old: old title
$nt: new title
$user: user who is doing the move
$err: error message
$reason: the reason for the move (added in 1.13)

'AbortNewAccount': Return false to cancel account creation.
$user: the User object about to be created (read-only, incomplete)
$message: out parameter: error message to display on abort

'AddNewAccount': after a user account is created
$user: the User object that was created. (Parameter added in 1.7)
$byEmail: true when account was created "by email" (added in 1.12)

'AjaxAddScript': Called in output page just before the initialisation
of the javascript ajax engine. The hook is only called when ajax
is enabled ( $wgUseAjax = true; ).

'AlternateEdit': before checking if an user can edit a page and
before showing the edit form ( EditPage::edit() ). This is triggered
on &action=edit.
$EditPage : the EditPage object

'APIEditBeforeSave': before saving a page with api.php?action=edit,
after processing request parameters. Return false to let the request
fail, returning an error message or an <edit result="Failure"> tag
if $resultArr was filled.
$EditPage : the EditPage object
$text : the new text of the article (has yet to be saved)
$resultArr : data in this array will be added to the API result

'APIQueryInfoTokens': use this hook to add custom tokens to prop=info.
Every token has an action, which will be used in the intoken parameter
and in the output (actiontoken="..."), and a callback function which
should return the token, or false if the user isn't allowed to obtain
it. The prototype of the callback function is func($pageid, $title)
where $pageid is the page ID of the page the token is requested for
and $title is the associated Title object. In the hook, just add
your callback to the $tokenFunctions array and return true (returning
false makes no sense)
$tokenFunctions: array(action => callback)

'APIQueryRevisionsTokens': use this hook to add custom tokens to prop=revisions.
Every token has an action, which will be used in the rvtoken parameter
and in the output (actiontoken="..."), and a callback function which
should return the token, or false if the user isn't allowed to obtain
it. The prototype of the callback function is func($pageid, $title, $rev)
where $pageid is the page ID of the page associated to the revision the
token is requested for, $title the associated Title object and $rev the
associated Revision object. In the hook, just add your callback to the
$tokenFunctions array and return true (returning false makes no sense)
$tokenFunctions: array(action => callback)

'ArticleAfterFetchContent': after fetching content of an article from the database
$article: the article (object) being loaded from the database
$content: the content (string) of the article

'ArticleDelete': before an article is deleted
$article: the article (object) being deleted
$user: the user (object) deleting the article
$reason: the reason (string) the article is being deleted
$error: if the deletion was prohibited, the (raw HTML) error message to display
  (added in 1.13)

'ArticleDeleteComplete': after an article is deleted
$article: the article that was deleted
$user: the user that deleted the article
$reason: the reason the article was deleted
$id: id of the article that was deleted

'ArticleEditUpdateNewTalk': before updating user_newtalk when a user talk page was changed
$article: article (object) of the user talk page

'ArticleEditUpdatesDeleteFromRecentchanges': before deleting old entries from recentchanges table, return false to not delete old entries
$article: article (object) being modified

'ArticleFromTitle': when creating an article object from a title object using Wiki::articleFromTitle()
$title: title (object) used to create the article object
$article: article (object) that will be returned

'ArticleInsertComplete': After a new article is created
$article: Article created
$user: User creating the article
$text: New content
$summary: Edit summary/comment
$isMinor: Whether or not the edit was marked as minor
$isWatch: (No longer used)
$section: (No longer used)
$flags: Flags passed to Article::doEdit()
$revision: New Revision of the article

'ArticleMergeComplete': after merging to article using Special:Mergehistory
$targetTitle: target title (object)
$destTitle: destination title (object) 

'ArticlePageDataAfter': after loading data of an article from the database
$article: article (object) whose data were loaded
$row: row (object) returned from the database server

'ArticlePageDataBefore': before loading data of an article from the database
$article: article (object) that data will be loaded
$fields: fileds (array) to load from the database

'ArticleProtect': before an article is protected
$article: the article being protected
$user: the user doing the protection
$protect: boolean whether this is a protect or an unprotect
$reason: Reason for protect
$moveonly: boolean whether this is for move only or not

'ArticleProtectComplete': after an article is protected
$article: the article that was protected
$user: the user who did the protection
$protect: boolean whether it was a protect or an unprotect
$reason: Reason for protect
$moveonly: boolean whether it was for move only or not

'ArticlePurge': before executing "&action=purge" 
$article: article (object) to purge

'ArticleRevisionVisiblitySet': called when changing visibility of one or more
revision of an article
&$title: title object of the article

'ArticleRevisionUndeleted': after an article revision is restored
$title: the article title
$revision: the revision
$oldPageID: the page ID of the revision when archived (may be null)

'ArticleRollbackComplete': after an article rollback is completed
$article: the article that was edited
$user: the user who did the rollback
$revision: the revision the page was reverted back to

'ArticleSave': before an article is saved
$article: the article (object) being saved
$user: the user (object) saving the article
$text: the new article text
$summary: the article summary (comment)
$isminor: minor flag
$iswatch: watch flag
$section: section #

'ArticleSaveComplete': After an article has been updated
$article: Article modified
$user: User performing the modification
$text: New content
$summary: Edit summary/comment
$isMinor: Whether or not the edit was marked as minor
$isWatch: (No longer used)
$section: (No longer used)
$flags: Flags passed to Article::doEdit()
$revision: New Revision of the article

'ArticleSaveComplete': after an article is saved
$article: the article (object) saved
$user: the user (object) who saved the article
$text: the new article text
$summary: the article summary (comment)
$isminor: minor flag
$iswatch: watch flag
$section: section #

wfRunHooks( 'ArticleSaveComplete', array( &$this, &$wgUser, $text, $summary, $flags & EDIT_MINOR, null, null, &$flags, $revision ) );

'ArticleUndelete': When one or more revisions of an article are restored
$title: Title corresponding to the article restored
$create: Whether or not the restoration caused the page to be created
(i.e. it didn't exist before)

'ArticleUpdateBeforeRedirect': After a page is updated (usually on save), before the user is redirected back to the page
&$article: the article
&$sectionanchor: The section anchor link (e.g. "#overview" )
&$extraq: Extra query parameters which can be added via hooked functions

'ArticleViewHeader': Before the parser cache is about to be tried for article viewing.
&$article: the article
&$pcache: whether to try the parser cache or not
&$outputDone: whether the output for this page finished or not

'ArticleViewRedirect': before setting "Redirected from ..." subtitle when follwed an redirect
$article: target article (object)

'AuthPluginAutoCreate': Called when creating a local account for an user logged
in from an external authentication method
$user: User object created locally

'AuthPluginSetup': update or replace authentication plugin object ($wgAuth)
Gives a chance for an extension to set it programattically to a variable class.
&$auth: the $wgAuth object, probably a stub

'AutopromoteCondition': check autopromote condition for user.
$type: condition type
$args: arguments
$user: user
$result: result of checking autopromote condition

'BadImage': When checking against the bad image list
$name: Image name being checked
&$bad: Whether or not the image is "bad"

Change $bad and return false to override. If an image is "bad", it is not
rendered inline in wiki pages or galleries in category pages.

'BeforeGalleryFindFile': before an image is fetched for a gallery
&$gallery,: the gallery object
&$nt: the image title 
&$time: image timestamp

'BeforePageDisplay': Prior to outputting a page
&$out: OutputPage object
&$skin: Skin object

'BeforeParserFetchTemplateAndtitle': before a template is fetched by Parser
&$parser: Parser object
&$title: title of the template
&$skip: skip this template and link it?
&$id: the id of the revision being parsed

'BeforeParserMakeImageLinkObj': before an image is rendered by Parser
&$parser: Parser object
&$nt: the image title
&$skip: skip this image and link it?
&$time: the image timestamp

'BeforeParserrenderImageGallery': before an image gallery is rendered by Parser
&$parser: Parser object
&$ig: ImageGallery object

'BeforeWatchlist': Override watchlist display or add extra SQL clauses.
$nondefaults: Assoc array with the following keys:
              days, hideOwn, hideBots, hideMinor, namespace
$wgUser: wgUser.
&$hookSql: a string which will be inserted without sanitation into the SQL query
           used to get the watchlist, at the end of the WHERE part.

'BlockIp': before an IP address or user is blocked
$block: the Block object about to be saved
$user: the user _doing_ the block (not the one being blocked)

'BlockIpComplete': after an IP address or user is blocked
$block: the Block object that was saved
$user: the user who did the block (not the one being blocked)

'BookInformation': Before information output on Special:Booksources
$isbn: ISBN to show information for
$output: OutputPage object in use

'BrokenLink': Before the HTML is created for a broken (i.e. red) link
&$linker: Linker instance
$nt: the page title
$query: the URL query string passed in
&$u: the URL of this link
&$style: the inline CSS style
&$prefix: a prefix prepended to the linked text
&$text: the text placed by the user in the wiki-link
&$inside: any additional alphanumeric characters placed after the wiki-link,
that are made part of the link text
&$trail: text placed immediately after the HTML link

'CategoryPageView': before viewing a categorypage in CategoryPage::view
$catpage: CategoryPage instance

'ChangesListInsertArticleLink': Override or augment link to article in RC list.
&$this:        ChangesList instance.
&$articlelink: HTML of link to article (already filled-in).
&$s:           HTML of row that is being constructed.
&$rc:          RecentChange instance.
$unpatrolled:  Whether or not we are showing unpatrolled changes.
$watched:      Whether or not the change is watched by the user.

'ContributionsToolLinks': Change tool links above Special:Contributions
$id: User identifier
$title: User page title
&$tools: Array of tool links

'CustomEditor': When invoking the page editor
$article: Article being edited
$user: User performing the edit

Return true to allow the normal editor to be used, or false
if implementing a custom editor, e.g. for a special namespace,
etc.

'DiffViewHeader': called before diff display
$diff: DifferenceEngine object that's calling
$oldRev: Revision object of the "old" revision (may be null/invalid)
$newRev: Revision object of the "new" revision

'DisplayOldSubtitle': before creating subtitle when browsing old versions of an article
$article: article (object) being viewed
$oldid: oldid (int) being viewed

'EditFilter': Perform checks on an edit
$editor: Edit form (see includes/EditPage.php)
$text: Contents of the edit box
$section: Section being edited
&$error: Error message to return
$summary: Edit summary for page

'EditFilterMerged': Post-section-merge edit filter
$editor: EditPage instance (object)
$text: content of the edit box
$error: error message to return
$summary: Edit summary for page

'EditFormPreloadText': Allows population of the edit form when creating new pages
&$text: Text to preload with
&$title: Title object representing the page being created

'EditPage::attemptSave': called before an article is
saved, that is before insertNewArticle() is called
&$editpage_Obj: the current EditPage object

'EditPage::showEditForm:fields': allows injection of form field into edit form
&$editor: the EditPage instance for reference
&$out: an OutputPage instance to write to
return value is ignored (should always return true)

'EditPage::showEditForm:initial': before showing the edit form
$editor: EditPage instance (object)

Return false to halt editing; you'll need to handle error messages, etc. yourself.
Alternatively, modifying $error and returning true will cause the contents of $error
to be echoed at the top of the edit form as wikitext. Return true without altering
$error to allow the edit to proceed.

'EditPageBeforeConflictDiff': allows modifying the EditPage object and output
when there's an edit conflict.  Return false to halt normal diff output; in
this case you're responsible for computing and outputting the entire "conflict"
part, i.e., the "difference between revisions" and "your text" headers and
sections.
&$editor: EditPage instance
&$out: OutputPage instance

'EditPageBeforeEditButtons':  allows modifying the edit buttons below the textarea in the edit form
&$editpage: The current EditPage object
&$buttons: Array of edit buttons "Save", "Preview", "Live", and "Diff"

'EditSectionLink': Override the return value of Linker::editSectionLink()
$skin: Skin rendering the UI
$title: Title being linked to
$section: Section to link to
$link: Default link
$result: Result (alter this to override the generated links)

'EditSectionLinkForOther': Override the return value of Linker::editSectionLinkForOther()
$skin: Skin rendering the UI
$title: Title being linked to
$section: Section to link to
$hint: Anchor title/tooltip attributes
$link: Default link
$result: Result (alter this to override the generated links)

'EmailConfirmed': When checking that the user's email address is "confirmed"
$user: User being checked
$confirmed: Whether or not the email address is confirmed
This runs before the other checks, such as anonymity and the real check; return
true to allow those checks to occur, and false if checking is done.

'EmailUser': before sending email from one user to another
$to: address of receiving user
$from: address of sending user
$subject: subject of the mail
$text: text of the mail

'EmailUserComplete': after sending email from one user to another
$to: address of receiving user
$from: address of sending user
$subject: subject of the mail
$text: text of the mail

'FetchChangesList': When fetching the ChangesList derivative for a particular user
&$user: User the list is being fetched for
&$skin: Skin object to be used with the list
&$list: List object (defaults to NULL, change it to an object instance and return
false override the list derivative used)

'FileDeleteComplete': When a file is deleted
$file: reference to the deleted file
$oldimage: in case of the deletion of an old image, the name of the old file
$article: in case all revisions of the file are deleted a reference to the article 
          associated with the file.
$user: user who performed the deletion
$reason: reason

'FileUpload': When a file upload occurs
$file : Image object representing the file that was uploaded

'FileUndeleteComplete': When a file is undeleted
$title: title object to the file
$fileVersions: array of undeleted versions. Empty if all versions were restored
$user: user who performed the undeletion
$reason: reason

'GetAutoPromoteGroups': When determining which autopromote groups a user is entitled to be in.
&$user: user to promote.
&$promote: groups that will be added.

'GetBlockedStatus': after loading blocking status of an user from the database
$user: user (object) being checked

'GetCacheVaryCookies': get cookies that should vary cache options
$out: OutputPage object
&$cookies: array of cookies name, add a value to it if you want to add a cookie
        that have to vary cache options

'GetFullURL': modify fully-qualified URLs used in redirects/export/offsite data
$title: Title object of page
$url: string value as output (out parameter, can modify)
$query: query options passed to Title::getFullURL()

'GetInternalURL': modify fully-qualified URLs used for squid cache purging
$title: Title object of page
$url: string value as output (out parameter, can modify)
$query: query options passed to Title::getInternalURL()

'GetLinkColours': modify the CSS class of an array of page links
$linkcolour_ids: array of prefixed DB keys of the pages linked to, indexed by page_id.
&$colours: (output) array of CSS classes, indexed by prefixed DB keys

'GetLocalURL': modify local URLs as output into page links
$title: Title object of page
$url: string value as output (out parameter, can modify)
$query: query options passed to Title::getLocalURL()

'getUserPermissionsErrors': Add a permissions error when permissions errors are
        checked for.  Use instead of userCan for most cases. Return false if the
        user can't do it, and populate $result with the reason in the form of
        array( messagename, param1, param2, ... ).  For consistency, error messages
        should be plain text with no special coloring, bolding, etc. to show that
        they're errors; presenting them properly to the user as errors is done by
        the caller.
$title: Title object being checked against
$user : Current user object
$action: Action being checked
$result: User permissions error to add. If none, return true.

'getUserPermissionsErrorsExpensive': Absolutely the same, but is called only
        if expensive checks are enabled.

'ImageBeforeProduceHTML': Called before producing the HTML created by a wiki
        image insertion.  You can skip the default logic entirely by returning
        false, or just modify a few things using call-by-reference.
&$this: Skin object
&$title: Title object of the image
&$file: File object, or false if it doesn't exist
&$frameParams: Various parameters with special meanings; see documentation in
        includes/Linker.php for Linker::makeImageLink2
&$handlerParams: Various parameters with special meanings; see documentation in
        includes/Linker.php for Linker::makeImageLink2
&$time: Timestamp of file in 'YYYYMMDDHHIISS' string form, or false for current
&$res: Final HTML output, used if you return false


'ImageOpenShowImageInlineBefore': Call potential extension just before showing
        the image on an image page
$imagePage: ImagePage object ($this)
$output: $wgOut

'ImagePageFileHistoryLine': called when a file history line is contructed
$file: the file
$line: the HTML of the history line
$css: the line CSS class

'ImagePageFindFile': called when fetching the file associated with an image page
$page: ImagePage object
&$file: File object
&$displayFile: displayed File object

'InitializeArticleMaybeRedirect': MediaWiki check to see if title is a redirect
$title: Title object ($wgTitle)
$request: WebRequest
$ignoreRedirect: boolean to skip redirect check
$target: Title/string of redirect target

'InitPreferencesForm': called at the end of PreferencesForm's constructor
$form: the PreferencesForm
$request: the web request to initialized from

'InternalParseBeforeLinks': during Parser's internalParse method before links but
after noinclude/includeonly/onlyinclude and other processing.
&$this: Parser object
&$text: string containing partially parsed text
&$this->mStripState: Parser's internal StripState object

'IsFileCacheable': Override the result of Article::isFileCacheable() (if true)
$article: article (object) being checked

'IsTrustedProxy': Override the result of wfIsTrustedProxy()
$ip: IP being check
$result: Change this value to override the result of wfIsTrustedProxy()

'isValidEmailAddr': Override the result of User::isValidEmailAddr(), for ins-
tance to return false if the domain name doesn't match your organization
$addr: The e-mail address entered by the user
&$result: Set this and return false to override the internal checks

'isValidPassword': Override the result of User::isValidPassword()
$password: The password entered by the user
&$result: Set this and return false to override the internal checks
$user: User the password is being validated for

'LanguageGetMagic': Use this to define synonyms of magic words depending of the language
$magicExtensions: associative array of magic words synonyms
$lang: laguage code (string)

'LanguageGetSpecialPageAliases': Use to define aliases of special pages names depending of the language
$specialPageAliases: associative array of magic words synonyms
$lang: laguage code (string)

'LinkerMakeExternalImage': At the end of Linker::makeExternalImage() just before the return
&$url: the image url
&$alt: the image's alt text
&$img: the new image HTML (if returning false)

'LinkerMakeExternalLink': At the end of Linker::makeExternalLink() just before the return
&$url: the link url
&$text: the link text
&$link: the new link HTML (if returning false)

'LinksUpdate': At the beginning of LinksUpdate::doUpdate() just before the actual update
&$linksUpdate: the LinkUpdate object

'LinksUpdateComplete': At the end of LinksUpdate::doUpdate() when updating has completed
&$linksUpdate: the LinkUpdate object

'LinksUpdateConstructed': At the end of LinksUpdate() is contruction.
&$linksUpdate: the LinkUpdate object

'LoadAllMessages': called by MessageCache::loadAllMessages() to load extensions messages

'LoadExtensionSchemaUpdates': called by maintenance/updaters.inc when upgrading database schema

'LocalFile::getHistory': called before file history query performed
$file: the file
$tables: tables
$fields: select fields
$conds: conditions
$opts: query options
$join_conds: JOIN conditions

'LoginAuthenticateAudit': a login attempt for a valid user account either succeeded or failed.
                          No return data is accepted; this hook is for auditing only.
$user: the User object being authenticated against
$password: the password being submitted and found wanting
$retval: a LoginForm class constant with authenticateUserData() return value (SUCCESS, WRONG_PASS, etc)

'LogLine': Processes a single log entry on Special:Log
$log_type: string for the type of log entry (e.g. 'move'). Corresponds to logging.log_type 
    database field.
$log_action: string for the type of log action (e.g. 'delete', 'block', 'create2'). Corresponds
    to logging.log_action database field.
$title: Title object that corresponds to logging.log_namespace and logging.log_title database fields.
$paramArray: Array of parameters that corresponds to logging.log_params field. Note that only $paramArray[0]
    appears to contain anything.
&$comment: string that corresponds to logging.log_comment database field, and which is displayed in the UI.
&$revert: string that is displayed in the UI, similar to $comment.
$time: timestamp of the log entry (added in 1.12) 

'LogPageValidTypes': action being logged. DEPRECATED: Use $wgLogTypes
&$type: array of strings

'LogPageLogName': name of the logging page(s). DEPRECATED: Use $wgLogNames
&$typeText: array of strings

'LogPageLogHeader': strings used by wfMsg as a header. DEPRECATED: Use $wgLogHeaders
&$headerText: array of strings

'LogPageActionText': strings used by wfMsg as a header. DEPRECATED: Use $wgLogActions
&$actionText: array of strings

'MagicWordMagicWords': When defining new magic word. DEPRECATED: Use LanguageGetMagic hook instead
$magicWords: array of strings

'MagicWordwgVariableIDs': When definig new magic words IDs. DEPRECATED: Use LanguageGetMagic hook instead
$variableIDs: array of strings

'MarkPatrolled': before an edit is marked patrolled
$rcid: ID of the revision to be marked patrolled
$user: the user (object) marking the revision as patrolled
$wcOnlySysopsCanPatrol: config setting indicating whether the user
 needs to be a sysop in order to mark an edit patrolled

'MarkPatrolledComplete': after an edit is marked patrolled
$rcid: ID of the revision marked as patrolled
$user: user (object) who marked the edit patrolled
$wcOnlySysopsCanPatrol: config setting indicating whether the user
 must be a sysop to patrol the edit

'MathAfterTexvc': after texvc is executed when rendering mathematics
$mathRenderer: instance of MathRenderer
$errmsg: error message, in HTML (string). Nonempty indicates failure
 of rendering the formula

'MediaWikiPerformAction': Override MediaWiki::performAction().
Use this to do something completely different, after the basic
globals have been set up, but before ordinary actions take place.
$output:  $wgOut
$article: $wgArticle
$title:   $wgTitle
$user:    $wgUser
$request: $wgRequest
$this:    The $mediawiki object

'MessagesPreLoad': When loading a message from the database
$title: title of the message (string)
$message: value (string), change it to the message you want to define

'MonoBookTemplateToolboxEnd': Called by Monobook skin after toolbox links have been rendered (useful for adding more)
Note: this is only run for the Monobook skin.  To add items to the toolbox
for all 'SkinTemplate'-type skins, use the SkinTemplateToolboxEnd hook
instead.
$tools: array of tools

'NewRevisionFromEditComplete': called when a revision was inserted due to an edit
$article: the article edited
$rev: the new revision
$baseID: the revision ID this was based off, if any

'OpenSearchUrls': Called when constructing the OpenSearch description XML.
Hooks can alter or append to the array of URLs for search & suggestion formats.
&$urls: array of associative arrays with Url element attributes

'OutputPageBeforeHTML': a page has been processed by the parser and
the resulting HTML is about to be displayed.  
$parserOutput: the parserOutput (object) that corresponds to the page 
$text: the text that will be displayed, in HTML (string)

'OutputPageParserOutput': after adding a parserOutput to $wgOut
$out: OutputPage instance (object)
$parserOutput: parserOutput instance being added in $out

'OutputPageMakeCategoryLinks': links are about to be generated for the page's categories.
  Implementations should return false if they generate the category links, so the default link generation is skipped.
$out: OutputPage instance (object)
$categories: associative array, keys are category names, values are category types ("normal" or "hidden")
$links: array, intended to hold the result. Must be an associative array with category types as keys and arrays of HTML links as values.

'PageHistoryBeforeList': When a history page list is about to be constructed.
$article: the article that the history is loading for

'PageHistoryLineEnding' : right before the end <li> is added to a history line
$row: the revision row for this line
$s: the string representing this parsed line

'PageHistoryPager::getQueryInfo': when a history pager query parameter set is constructed
$pager: the pager
$queryInfo: the query parameters

'PageRenderingHash': alter the parser cache option hash key
  A parser extension which depends on user options should install
  this hook and append its values to the key.
$hash: reference to a hash key string which can be modified

'ParserAfterStrip': Same as ParserBeforeStrip

'ParserAfterTidy': Called after Parser::tidy() in Parser::parse()
$parser: Parser object being used 
$text: text that'll be returned

'ParserBeforeInternalParse': called at the beginning of Parser::internalParse()
$parser: Parser object
$text: text to parse
$stripState: StripState instance being used

'ParserBeforeStrip': Called at start of parsing time (no more strip, deprecated ?)
$parser: parser object
$text: text being parsed
$stripState: stripState used (object)

'ParserBeforeTranscludeTemplate': Allows modification of the text and title of a
  template which is being transcluded
&$parser: parser object
&$text: text being transcluded
&$title: title object of the template

'ParserBeforeTidy': called before tidy and custom tags replacements
$parser: Parser object being used 
$text: actual text

'ParserClearState': called at the end of Parser::clearState()
$parser: Parser object being cleared

'ParserFirstCallInit': called when the parser initialises for the first time
&$parser: Parser object being cleared

'ParserGetVariableValueSwitch': called when the parser need the value of a custom magic word
$parser: Parser object
$varCache: array to store the value in case of multiples calls of the same magic word
$index: index (string) of the magic
$ret: value of the magic word (the hook should set it)

'ParserGetVariableValueTs': use this to change the value of the time for the {{LOCAL...}} magic word
$parser: Parser object
$time: actual time (timestamp)

'ParserGetVariableValueVarCache': use this to change the value of the variable cache or return false to not use it
$parser: Parser object
$varCache: varaiable cache (array)

'ParserLimitReport': called at the end of Parser:parse() when the parser will include comments about size of the text parsed
$parser: Parser object
$limitReport: text that will be included (without comment tags)

'ParserMakeImageParams': Called before the parser make an image link, use this to modify the parameters of the image.
$title: title object representing the file
$file: file object that will be used to create the image
&$params: 2-D array of parameters

'ParserTestParser': called when creating a new instance of Parser in maintenance/parserTests.inc
$parser: Parser object created

'ParserTestTables': alter the list of tables to duplicate when parser tests
are run. Use when page save hooks require the presence of custom tables
to ensure that tests continue to run properly.
&$tables: array of table names

'PersonalUrls': Alter the user-specific navigation links (e.g. "my page,
my talk page, my contributions" etc).

&$personal_urls: Array of link specifiers (see SkinTemplate.php)
&$title: Title object representing the current page

'PingLimiter': Allows extensions to override the results of User::pingLimiter()
&$user : User performing the action
$action : Action being performed
&$result : Whether or not the action should be prevented
Change $result and return false to give a definitive answer, otherwise
the built-in rate limiting checks are used, if enabled.

'PreferencesUserInformationPanel': Add HTML bits to user information list in preferences form
$form : PreferencesForm object
&$html : HTML to append to

'PrefixSearchBackend': Override the title prefix search used for OpenSearch and
AJAX search suggestions. Put results into &$results outparam and return false.
$ns : array of int namespace keys to search in
$search : search term (not guaranteed to be conveniently normalized)
$limit : maximum number of results to return
&$results : out param: array of page names (strings)

'PrefsEmailAudit': called when user changes his email address
$user: User (object) changing his email address
$oldaddr: old email address (string)
$newaddr: new email address (string)

'PrefsPasswordAudit': called when user changes his password
$user: User (object) changing his passoword
$newPass: new password
$error: error (string) 'badretype', 'wrongpassword', 'error' or 'success'

'RawPageViewBeforeOutput': Right before the text is blown out in action=raw
&$obj: RawPage object
&$text: The text that's going to be the output

'RecentChange_save': called at the end of RecenChange::save()
$recentChange: RecentChange object

'RenderPreferencesForm': called at the end of PreferencesForm::mainPrefsForm
$form: the PreferencesForm
$out: output page to render to, probably $wgOut

'ResetPreferences': called at the end of PreferencesForm::resetPrefs
$form: the PreferencesForm
$user: the User object to load preferences from

'RevisionInsertComplete': called after a revision is inserted into the DB
$revision: the Revision

'SavePreferences': called at the end of PreferencesForm::savePreferences; 
                   returning false prevents the preferences from being saved.
$form: the PreferencesForm
$user: the User object to save preferences to
$message: change this to set an error message (ignored if the hook does not return false)
$old: old preferences of the user

'SearchUpdate': Prior to search update completion
$id : Page id
$namespace : Page namespace
$title : Page title
$text : Current text being indexed

'SearchGetNearMatch': An extra chance for exact-title-matches in "go" searches
$term : Search term string
&$title : Outparam; set to $title object and return false for a match

'ShowRawCssJs': Customise the output of raw CSS and JavaScript in page views
$text: Text being shown
$title: Title of the custom script/stylesheet page
$output: Current OutputPage object

'SiteNoticeBefore': Before the sitenotice/anonnotice is composed
&$siteNotice: HTML returned as the sitenotice
Return true to allow the normal method of notice selection/rendering to work,
or change the value of $siteNotice and return false to alter it.

'SiteNoticeAfter': After the sitenotice/anonnotice is composed
&$siteNotice: HTML sitenotice
Alter the contents of $siteNotice to add to/alter the sitenotice/anonnotice.

'SkinAfterBottomScripts': At the end of Skin::bottomScripts()
$skin: Skin object
&$text: bottomScripts Text
Append to $text to add additional text/scripts after the stock bottom scripts.

'SkinSubPageSubtitle': At the beginning of Skin::subPageSubtitle()
$skin: Skin object
&$subpages: Subpage links HTML
If false is returned $subpages will be used instead of the HTML subPageSubtitle() generates.
If true is returned, $subpages will be ignored and the rest of subPageSubtitle() will run.

'SkinTemplateBuildContentActionUrlsAfterSpecialPage': after the single tab  when showing a special page
$sktemplate: SkinTemplate object
$content_actions: array of tabs

'SkinTemplateBuildNavUrlsNav_urlsAfterPermalink': after creating the "permanent link" tab
$sktemplate: SkinTemplate object
$nav_urls: array of tabs

'SkinTemplateContentActions': Alter the "content action" links in SkinTemplates
&$content_actions: Content actions
[See http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/examples/Content_action.php
for an example]

'SkinTemplateOutputPageBeforeExec': Before SkinTemplate::outputPage() starts page output
&$sktemplate: SkinTemplate object
&$tpl: Template engine object

'SkinTemplatePreventOtherActiveTabs': use this to prevent showing active tabs
$sktemplate: SkinTemplate object
$res: set to true to prevent active tabs

'SkinTemplateSetupPageCss': use this to provide per-page CSS
$out: Css to return

'SkinTemplateTabAction': Override SkinTemplate::tabAction().
       You can either create your own array, or alter the parameters for the normal one.
&$this:     The SkinTemplate instance.
$title:     Title instance for the page.
$message:   Visible label of tab.
$selected:  Whether this is a selected tab.
$checkEdit: Whether or not the action=edit query should be added if appropriate.
&$classes:  Array of CSS classes to apply.
&$query:    Query string to add to link.
&$text:     Link text.
&$result:   Complete assoc. array if you want to return true.

'SkinTemplateTabs': called when finished to build the actions tabs
$sktemplate: SkinTemplate object
$content_actions: array of tabs

'SkinTemplateToolboxEnd': Called by SkinTemplate skins after toolbox links have been rendered (useful for adding more)
$tools: array of tools

'SpecialContributionsBeforeMainOutput': Before the form on Special:Contributions
$id: User identifier

'SpecialListusersDefaultQuery': called right before the end of UsersPager::getDefaultQuery()
$pager: The UsersPager instance
$query: The query array to be returned

'SpecialListusersFormatRow': called right before the end of UsersPager::formatRow()
$item: HTML to be returned. Will be wrapped in <li></li> after the hook finishes
$row: Database row object

'SpecialListusersHeader': called before closing the <fieldset> in UsersPager::getPageHeader()
$pager: The UsersPager instance
$out: The header HTML

'SpecialListusersHeaderForm': called before adding the submit button in UsersPager::getPageHeader()
$pager: The UsersPager instance
$out: The header HTML

'SpecialListusersQueryInfo': called right before the end of UsersPager::getQueryInfo()
$pager: The UsersPager instance
$query: The query array to be returned

'SpecialMovepageAfterMove': called after moving a page
$movePage: MovePageForm object
$oldTitle: old title (object)
$newTitle: new title (object)

'SpecialPageExecuteAfterPage': called after executing a special page
Warning: Not all the special pages call this hook
$specialPage: SpecialPage object
$par: paramter passed to the special page (string)
$funct: function called to execute the special page

'SpecialPageExecuteBeforeHeader': called before setting the header text of the special page
Warning: Not all the special pages call this hook
$specialPage: SpecialPage object
$par: paramter passed to the special page (string)
$funct: function called to execute the special page

'SpecialPageExecuteBeforePage': called after setting the special page header text but before the main execution
Warning: Not all the special pages call this hook
$specialPage: SpecialPage object
$par: paramter passed to the special page (string)
$funct: function called to execute the special page

'SpecialPage_initList': called when setting up SpecialPage::$mList, use this hook to remove a core special page
$list: list (array) of core special pages

'SpecialRecentChangesPanel': called when building form options in SpecialRecentChanges
&$extraOpts: array of added items, to which can be added
$opts: FormOptions for this request

'SpecialRecentChangesQuery': called when building sql query for SpecialRecentChanges
&$conds: array of where conditionals for query
&$tables: array of tables to be queried
&$join_conds: join conditions for the tables
$opts: FormOptions for this request

'SpecialSearchNogomatch': called when user clicked the "Go" button but the target doesn't exist
$title: title object generated from the text entred by the user

'SpecialSearchResults': called before search result display when there are matches
$term: string of search term
&$titleMatches: empty or SearchResultSet object
&$textMatches: empty or SearchResultSet object

'SpecialSearchNoResults': called before search result display when there are no matches
$term: string of search term

'SpecialVersionExtensionTypes': called when generating the extensions credits, use this to change the tables headers
$extTypes: associative array of extensions types

'TitleMoveComplete': after moving an article (title)
$old: old title
$nt: new title
$user: user who did the move
$pageid: database ID of the page that's been moved
$redirid: database ID of the created redirect

'UndeleteShowRevision': called when showing a revision in Special:Undelete
$title: title object related to the revision
$rev: revision (object) that will be viewed

'UnknownAction': An unknown "action" has occured (useful for defining
                 your own actions)
$action: action name
$article: article "acted on"

'UnwatchArticle': before a watch is removed from an article
$user: user watching
$article: article object to be removed

'UnwatchArticle': after a watch is removed from an article
$user: user that was watching
$article: article object removed

'UnwatchArticleComplete': after a watch is removed from an article
$user: user that watched
$article: article object that was watched

'UploadForm:initial': before the upload form is generated
$form: UploadForm object
You might set the member-variables $uploadFormTextTop and 
$uploadFormTextAfterSummary to inject text (HTML) either before
or after the editform.

'UploadForm:BeforeProcessing': at the beginning of processUpload()
$form: UploadForm object
Lets you poke at member variables like $mUploadDescription before the
file is saved.

'UploadVerification': additional chances to reject an uploaded file
string $saveName: destination file name
string $tempName: filesystem path to the temporary file for checks
string &$error: output: HTML error to show if upload canceled by returning false

'UploadComplete': Upon completion of a file upload
$uploadForm: Upload form object. File can be accessed by $uploadForm->mLocalFile.

'UserArrayFromResult': called when creating an UserArray object from a database result
&$userArray: set this to an object to override the default object returned
$res: database result used to create the object

'userCan': To interrupt/advise the "user can do X to Y article" check.
        If you want to display an error message, try getUserPermissionsErrors.
$title: Title object being checked against
$user : Current user object
$action: Action being checked
$result: Pointer to result returned if hook returns false. If null is returned,
 userCan checks are continued by internal code.

'UserCanSendEmail': To override User::canSendEmail() permission check
$user: User (object) whose permission is being checked
&$canSend: bool set on input, can override on output


'UserClearNewTalkNotification': called when clearing the "You have new messages!" message, return false to not delete it
$user: User (object) that'll clear the message

'UserCreateForm': change to manipulate the login form
$template: SimpleTemplate instance for the form

'UserEffectiveGroups': Called in User::getEffectiveGroups()
$user: User to get groups for
&$groups: Current effective groups

'UserGetAllRights': after calculating a list of all available rights
&$rights: Array of rights, which may be added to.

'UserGetEmail': called when getting an user email address
$user: User object
&$email: email, change this to override local email

'UserGetEmailAuthenticationTimestamp': called when getting the timestamp of email authentification
$user: User object
&$timestamp: timestamp, change this to override local email authentification timestamp

'UserGetImplicitGroups': Called in User::getImplicitGroups()
&$groups: List of implicit (automatically-assigned) groups

'UserGetRights': Called in User::getRights()
$user: User to get rights for
&$rights: Current rights

'UserLoadDefaults': called when loading a default user
$user: user object
$name: user name

'UserLoadFromSession': called to authenticate users on external/environmental means
$user: user object being loaded
&$result: set this to a boolean value to abort the normal authentification process

'UserLoginComplete': after a user has logged in
$user: the user object that was created on login
$inject_html: Any HTML to inject after the "logged in" message.
                    
'UserLoginForm': change to manipulate the login form
$template: SimpleTemplate instance for the form

'UserLogout': before a user logs out
$user: the user object that is about to be logged out
       
'UserLogoutComplete': after a user has logged out
$user: the user object _after_ logout (won't have name, ID, etc.)
$inject_html: Any HTML to inject after the "logged out" message.
$oldName: name of the user before logout (string)

'UserRights': After a user's group memberships are changed
$user  : User object that was changed
$add   : Array of strings corresponding to groups added
$remove: Array of strings corresponding to groups removed

'UserRetrieveNewTalks': called when retrieving "You have new messages!" message(s)
$user: user retrieving new talks messages
$talks: array of new talks page(s)

'UserSaveSettings': called when saving user settings
$user: User object

'UserSetCookies': called when setting user cookies
$user: User object
&$session: session array, will be added to $_SESSION
&$cookies: cookies array mapping cookie name to its value

'UserSetEmail': called when changing user email address
$user: User object
&$email: new email, change this to override new email address

'UserSetEmailAuthenticationTimestamp': called when setting the timestamp of email authentification
$user: User object
&$timestamp: new timestamp, change this to override local email authentification timestamp

'UserToggles': called when initialising User::$mToggles, use this to add new toggles
$toggles: array of toggles to add

'WatchArticle': before a watch is added to an article
$user: user that will watch
$article: article object to be watched

'WatchArticleComplete': after a watch is added to an article
$user: user that watched
$article: article object watched

'wgQueryPages': called when initialising $wgQueryPages, use this to add new query pages to be updated with maintenance/updateSpecialPages.php
$query: $wgQueryPages itself

More hooks might be available but undocumented, you can execute
./maintenance/findhooks.php to find hidden one.