Authentication system?

Hello All, I had written a user authentication/management system w/PHP & MySQL several years back and while we still have the code to potentially use a springboard to go forward, you know how sometimes it's just not the same as modern code per say. I wanted to see if maybe there is an open source, well managed app/code out there which does the same thing that we may be better off using rather then cleaning up old code. Here is what we are trying to accomplish: System that manages users to gain access to whatever custom app we want. System Features: Register Get approved (manual process) Notifications on registration, approval or denial of access. Password recovery Ability to have sub users managed by the main user account. Management: Accept or deny registration requests See a login log See a management log of who approved/denied users. Manage active users. I appreciate your help! -- Thanks! Joey