I am using the PHP library for the Graph API (http://github.com/facebook/php-sdk) but am a bit confused about how it is all working (or not).

I just want to authenticate the user and get the user ID back. What I really want to know is what to do once the user has logged in to Facebook and returned to my site. There is data called 'session' in the URL. Does this need to be stored in order to continually get the user ID? It is really not apparent, to me, from the samples or (lack of) documentation.

Also, would it be easier to just cut out the PHP library altogether and just handle the reponse myself, storing it in a session variable. If i were to do this, what is the best method of getting/extracting the current users ID?