Session Authentication etc...

I'm currently rewriting a website in PHP and something I'm looking to do is login and logouts but also have a page to view who is logged at the present time. I've kinda made some login code which basically checks the database and then sets a cookie which I think I will change cause it looks like I wonna do session variables. Anways anyone either know of a decent authentication lib that I can use to check the users online and do authentication or can someone point me in the right direction in doing this? Thanks Troy