Use PHP's implode() with the suggested order of arguments

Use PHP's implode() with the suggested order of arguments

https://secure.php.net/manual/en/function.implode.php defines the order
of arguments as

 string implode ( string $glue , array $pieces )
 string implode ( array $pieces )

Note:
  implode() can, for historical reasons, accept its parameters in
  either order. For consistency with explode(), however, it may be less
  confusing to use the documented order of arguments.

Change-Id: I03bf5712204e283f52d3ede54af9b9ec117d4280