What should I use for session_id for a facebok IFrame app?

Question

I'm working on an IFrame apps in Facebook.
I use the PHP SDK provided in facebook. and I can login. However, what should I use for session id in PHP? should I use the default PHP session id (by the server)? or should I use the 'sig' that I got from the facebook SDK?

(i.e. the $session = $facebook->getSession() , $session['sig'] )

I'm confused by the getSession() from that PHP SDK, and what should be set i my server.