I managed to develop a PHP authentication/activation form which works fine for what I need, now what I'd like to do is adding an option to permit these members to become premium members by paying a fee so that they can fully access to the website contents (..i know it sounds like a "xxx" website... ;D).
Do you know any book or tutorials where I can find and learn a basic way on how to do it?
What you're looking for is a role based access system.
You may have three types of users, e.g. guest, user and premium.
In your user object, you will have to record what role the user is, and then for any page, check which roles can access.
This is often implemented using an ACL, and the Zend Framework has a very good one. http://framework.zend.com/manual/en/zend.acl.html