public function queueRequest($handle, $url, $callback)
    {
        if (!is_resource(self::$queue_handle)) {
            self::initQueue();
        }

        self::$request_queue[] = array(
            'url'       => $url,
            'handle'    => $handle,
            'callback'  => $callback,
            'response'  => '',
        );
        curl_multi_add_handle(self::$queue_handle, $handle);
    }