@phpdeveloper.org
Chris Shiflet has added to the articles section of his site with a new post dealing with keeping sessions in databases.
I've added another free article to my growing collection: Guru Speak: Storing Sessions in a Database
This article creates the necessary functions in steps, so that you hopefully better understand the final product and can make your own modifications to better suit your needs.
He also provides a link to just the code if you don't want to work through the article too. He shares another custom session handling link as well - one from the Zend Developer Zone called "Trick Out Your Session Handler".