public function getRequestToken($oauth_callback = null)
    {
        $parameters = [];
        if (!empty($oauth_callback)) {
            $parameters['oauth_callback'] = $oauth_callback;
        }
        $code = $this->request('POST', $this->url('oauth/request_token', ''), $parameters);
        if (isset($this->response['code']) && $this->response['code'] == 200 && !empty($this->response['response'])) {
            $get_parameters = $this->response['response'];
            $token = [];
            parse_str($get_parameters, $token);
        }
        // Return the token if it was properly retrieved
        if (isset($token['oauth_token'], $token['oauth_token_secret'])) {
            return $token;
        } else {
            throw new \Exception('No request token found.');
        }
    }