private function send_http($url, $data, $headers=[])
    {
        if ($this->curl_method == 'async') {
            $this->_curl_handler->enqueue($url, $data, $headers);
        } elseif ($this->curl_method == 'exec') {
            $this->send_http_asynchronous_curl_exec($url, $data, $headers);
        } else {
            $this->send_http_synchronous($url, $data, $headers);
        }
    }