/**
  * Returns an instance of the enabled user auth class.
  * 
  * @return	UserAuth
  */
 public static function getInstance()
 {
     if (self::$instance === null) {
         // call loadInstance event
         if (!defined('NO_IMPORTS')) {
             EventHandler::fireAction('UserAuth', 'loadInstance');
         }
         if (self::$instance === null) {
             self::$instance = new UserAuthDefault();
         }
     }
     return self::$instance;
 }