uLogin is a PHP library for developing secure login and authentication systems. Provides secured user sessions, password storage and logins for multiple user databases. It uses multiple measures to counter online and offline attacks.
After a very deep analysis of at least a dozen of the best frameworks, I chose this. It is among the easiest to integrate, but pays great attention to security and scalability. The code is commented, but it would be appropriate to provide him with some more examples (i.e.: password recovery email with persistent nonce, registration with mail confirmation, double opt-in, password changes, etc.). WARNING: very poor developer support!
I will definitely uses this for more of my apps.