Sessions & Headers [AGAIN!]

Ok sorry to ask this again, but from searching my archives there are a few people who've asked this, but never gotten a good answer. Is there a means to be able to check for a valid session id - checking if it is already active in the system. If there is none - rather than immediately starting a session (as is the default behavior of PHP) then allow another action - such as a log-in screen via a http redirect - which if successful then a new session id is assigned. It would seem a straight forward enough task - for anyone who understood the internals of PHP's sessions handling. Possibly there is even an array of all existing session ID living within PHP that we could work with. Or - by default the session information is saved to a text file - if none of the session information is stored within PHP as an accessible array or object (or something) - then maybe we can read the text file, parse out the session id's and then store them in an array for this purpose. Of course at that point it starts to sound like a lot of work. :(