It sounds like what you want is a user control library, rather than an authentication library.
For example, in the Zend Framework there are two classes: Zend_Auth (which handles user authentication: logins (e.g. simple database tables to OpenID)) and Zend_Acl (which handles the user access side of things).
I quite like the ZF classes - I haven't tried using them outside of a ZF project but most of their classes can so give it a try. Even if you decide to build your own they'd be useful for reference.
