Oldy but Goody
phpSecureSite is a modular authentication and session-handling system for PHP, released under the GNU GPL license. It is primarily intended for use in closed web-applications, where users are required to enter a username and password to gain access. It is made to be fully integrated with the web-application, so that users will not notice that you are using a separate authentication and session system. This means that you must code the front-end (such as login screens) yourself. More information can be found on the about page.