session vars set in functions invalid??

Hi all, I am trying, as I'm sure many others are, to come up with a compact user authentication/login scheme. It seems to me that for my purposes, the best way to go is to run a function which receives the username and supplied password. A lookup in my database is performed, and a session variable is set which tells me who the user is. My database lookup works just fine. However, It appears that session variable scope doesn't penetrate outside of functions. Can this be? I thought the whole point of session variables was that they are accessible from anywhere. If I can't set a session variable within a function, then it appears that I have to in-line code the authentication process wherever it is needed, which seems rather silly to me. Unless I can get some clarity on this, it seems I will have to roll my own session handling using a temporary database table. Can anyone give me some help to spare me from this work? Thanks in advance, michael geary