Manual:$wgHooks

Details
Global list of hooks. It is an array of arrays: it specifies for every event an array of functions to be called.

Add a hook by doing: $wgHooks['event_name'][] = 'function'; or: $wgHooks['event_name'][] = array('function', $data); or: $wgHooks['event_name'][] = array($object, 'method');

See Manual:MediaWiki hooks for details.