phpMyID is a single user (though, if you were so inclined, you could easily turn it into a multi-user setup) IdP, or "Identity Provider" for the OpenID framework. It's a single PHP script with minimal dependancies. You don't need a database, you don't need to make your filesystem writable, you don't need to download any libraries, and you don't need to recompile PHP. Okay, well, you shouldn't need to do any of that.

You can get all of the details on the project and the latest downloads from their website including how to install and use the tool.