Manual:Hooks/getUserPermissionsErrorsExpensive

Details

 * $title: Title object being checked against
 * $user : Current user object
 * $action: Action being checked
 * &$result: User permissions error to add. If none, return true.

Differences from getUserPermissionsErrors
Both hooks are typically run when checking for proper permissions in Title.php. When it is desireable to skip potentially expensive cascading permission checks, only getUserPermissionsErrors is run. This behavior is suitable for nonessential UI controls in common cases, but not for functional access control. This behavior may provide false positives, but should never provide a false negative.