public function get_access_token($token, $secret, $verifier='') {
        $this->sign_secret = $this->consumer_secret.'&'.$secret;
        $params = $this->prepare_oauth_parameters($this->access_token_api, array('oauth_token'=>$token, 'oauth_verifier'=>$verifier), 'POST');
        $this->setup_oauth_http_header($params);
        // Should never send the callback in this request.
        unset($params['oauth_callback']);
        $content = $this->http->post($this->access_token_api, $params, $this->http_options);
        $keys = $this->parse_result($content);

        if (empty($keys['oauth_token']) || empty($keys['oauth_token_secret'])) {
            throw new moodle_exception('oauth1accesstoken', 'core_error', '', null, $content);
        }

        $this->set_access_token($keys['oauth_token'], $keys['oauth_token_secret']);
        return $keys;
    }