In a pointer from PHPKitchen.com today, there's a new tutorial discussing the use of PEAR's LiveUser library to help you authenticate and add/remove/update permissions for users of your site.

LiveUser's purpose is to provide a flexible and extensible framework to be used for authentication and authorization schemes. It does not aim at being the "One-true-solution" but rather providing the means to achieve what you need.

[...] LiveUser's design is based on containers. A container is similar in concept to a driver. Containers are the same, to handle a specific authentication scheme you need a container. The design is based on two sets of containers. One set is used for authentication and the other for permissions.

LiveUser definitely seems like a felxible package to aid in the authentication/permissions of your site - and this tutorial can definitely help you get on your way. They cover everything from the install of the package all the way out to a sample application, complete with code.