I would look into what sort of cookies get set via the other applications, and model your authentication piece accordingly.
I use a simple SetCookie to establish whatever login they use in $login, then bounce that against a db on every page I want the user to have access to. If the cookie gets deleted, or they logout (which nullifies the cookie) and try to return, it redirects them to the login page.
If the other systems use something similar, you just need to see what gets set.
