/**
    *  判定是否ajax调用
    */
   public static function is_ajax()
   {
       if (isset($_SERVER['HTTP_X_REQUESTED_WITH']))
       {
           // Typically used to denote AJAX requests
           $requested_with = strtolower($_SERVER['HTTP_X_REQUESTED_WITH']);
       }
       return ($requested_with === 'xmlhttprequest');
   }