+
+ /**
+ * Generate a srcset attribute value from an array mapping pixel densities
+ * to URLs. Note that srcset supports width and height values as well, which
+ * are not used here.
+ *
+ * @param array $urls
+ * @return string
+ */
+ static function srcSet( $urls ) {
+ $candidates = array();
+ foreach( $urls as $density => $url ) {
+ // Image candidate syntax per current whatwg live spec, 2012-09-23:
+ // http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content-1.html#attr-img-srcset
+ $candidates[] = "{$url} {$density}x";
+ }
+ return implode( ", ", $candidates );
+ }