creating a userdatabase with groups in PHP/MySQL

Hello, I have to make a loginprocedure for our intranet at my work. I have to create a userdatabase with users and usergroups ( like Windows NT/2000 User Manager and Novell). Before I start with this job I wonder if maybe somebody created already something like this in PHP/MySQL and I can just use it. (This user database is not the security from MySQL! but just a seperated userbase wich will look to a authorisationtable to determine WHO can enter WHAT part of the Intranet.) Or if you have some advise for me to create something like this, I'll be very thankfull! With greetings, Sander Peters The Netherlands