public function createIdToken($client_id, $userInfo, $nonce = null, $userClaims = null, $access_token = null)
    {
        // pull auth_time from user info if supplied
        list($user_id, $auth_time) = $this->getUserIdAndAuthTime($userInfo);

        $token = array(
            'iss'        => $this->config['issuer'],
            'sub'        => $user_id,
            'aud'        => $client_id,
            'iat'        => time(),
            'exp'        => time() + $this->config['id_lifetime'],
            'auth_time'  => $auth_time,
        );

        if ($nonce) {
            $token['nonce'] = $nonce;
        }

        if ($userClaims) {
            $token += $userClaims;
        }

        if ($access_token) {
            $token['at_hash'] = $this->createAtHash($access_token, $client_id);
        }

        return $this->encodeToken($token, $client_id);
    }