Are you creating some kind of web application in which you need people to register and so on? Well if so, UserCake will be a good stating point:

“UserCake is a simple user management system developed in PHP. For sometime now people have asked me where can I find a simple, flexible and most of all customisable user management system. I never seem to have an appropriate answer to hand. There are many user systems on the internet and a lot of tutorials too. Sadly many are convoluted, poorly written and insecure. The goal of UserCake is to provide people with a foundation to build user applications without all the hassle. UserCake is cleanly written, well commented, formatted and documented, did I forget to mention it’s object oriented for easy customisation? Simple codebase means developing with UserCake is straightforward and most importantly fast. UserCake is fully opensource, you may download and re-distribute the code in any form, a link back to the project would be nice, but not required. Enjoy!”

Actually I am integrating it with a Flex application, and it looks very cool! Hope I manage to release a sort of FlexCake sooner or later..