Zend_OpenId is compatible with PHP 5.3, however it only supports OpenID 1.1 and will not work with Google (which uses OpenID 2.0). Janrain's OpenID system normally available via http://openidenabled.com/php-openid/ is the one I'm using for my PHP 5 application (the OpenID enabled site is currently slightly down for maintenance today) - it is also one of the more popular PHP OpenID implementations (only small snag is that it uses temporary files for the OpenID sessions - making getting it working on a multi-server environment slightly tricky).
