code: public static function destroy () {
          if(self::$started) {
              self::$started = false;
              unset($_COOKIE[self::$cookieName]);
              setcookie(self::$cookieName, '', 1, '/');
              session_destroy();
          } else {
              trigger_error('Session is not started!', E_USER_WARNING);
          }
      }
docs: Destroy session