public function createAuthorizationCode($client_id, $user_id, $redirect_uri, $scope = null)
    {
        $code = $this->generateAuthorizationCode();
        $this->storage->setAuthorizationCode($code, $client_id, $user_id, $redirect_uri, time() + $this->config['auth_code_lifetime'], $scope);

        return $code;
    }