If the project is going to be PHP based, why not just handle the user management component of the project on the PHP side, then authenticate to filemaker using a group privilege assigned via their PHP table user record?
There are plenty of PHP packages our there (EG usercake) that support the ability to assign privilege levels, and are already robustly coded and free. There's also relatively cheap packages (~$20) that support SSO via facebook and such.
Sweet! I like that idea.
Will that let me limit access on the Account Level?
Another consideration, if you keep any creation/modification user information in FileMaker, you'll have to pass the values manually via PHP, which should be easy given that the login systems all give you session variables that can easily be used.