private function _request()
    {
        $res = $this->_client->request(
            $this->_method,
            $this->_endpoint . '?' . $this->_query_parameters,
            [
                'headers' => $this->_headers,
                'http_errors' => false
            ]
        );
        return array(
            'code' => $res->getStatusCode(),
            'body' => new SimpleXMLElement($res->getBody())
        );
    }