Manual:Hooks/MediaWikiPerformAction

Details
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

Note: To prevent the standard performAction class from doing anything, return false from this hook.