From GigaSpartan.com:
In this tutorial I will be showing you how to make a simple login system consisting of a login page, register page, forgotten password page, email activation, logout page and finally a users online page. I made this tutorial to mainly target new-to-PHP developers, due to the fact when I started I noticed the lack in quantity of basic login systems. Therefore, I decided to make one myself giving high quality advice on how to make your first login system with a users online script!
Tutorial link: http://gigaspartan.com/2010/11/26/how-to-build-a-full-featured-login-system/