Though I have not used it, CakePHP offers user management and access control list functionality, backed by other helpers that take care of sanitizing input. I'm not sure if an MVC framework is ideal for your needs, though, as it would steepen your learning curve.
I'm sure other answers will recommend 'pluggable' user authentication and ACL classes, which will accrue votes based on how useful other people found them.
I rolled my own a long time ago and tend to use it in lighter MVC frameworks like Code Igniter, or in the absence of any framework at all. As you use ready made classes, you'll naturally and quickly begin developing your own to better suit your needs.
