PHP / MySQL Authentication class

Can anyone point me to a good User Authentication class written in PHP (Version 5 preferably) that uses MySQL as the backend. If the output is valid XHTML then even better. There are lots of hits on Google but I need some help sorting the wheat from the chaff. It's for a hobbyist site so I'm looking for something using cookies that will keep people logged in between visits and provide some personalisation. Obviously it needs to be reasonably secure but it's not a banking system or anything. Thanks! Alan