Trying to integrate a number of different PHP based application into something where everyone rolls there own authentication is quite a challenge. One easy way to do it is to use the REMOTE_USER sent by a front end authentication server. This way, any authentication mechanism that can be implemented ahead of PGV is available.
This solution is used in for example PHPWebGallery http://www.phpwebgallery.net/ and work very well.
By accepting any REMOTE_USER and create an entry in the database the first time with guest/unauthenticated rights it's possible to administer the user with the usual tools and add group membership etc. to elevate the rights of the user after identification. Of cause this behavior is controlled by a configuration knob.
Using this mechanism and mod_auth_openid http://trac.butterfat.net/public/mod_auth_openid I can use OpenID with the unchanged application.
/glz
Logged In: YES
user_id=634811
Originator: NO
Moving to the Feature Request section.