public static function getInstance() { if (null == self::$_instance) { $session = new Zend_Session_Namespace(self::SESSION_ALIAS); if (null == $session->acl) { self::$_instance = new self(); $session->acl = serialize(self::$_instance); } else { self::$_instance = unserialize($session->acl); } } return self::$_instance; }