title: Detect if PHP session exists
body: <p>Facebook now offer subscriptions to users so you can get realtime updates on changes. If my app receives an update, I plan to store it in the database. I would also like to detect if their session exists. If it does then I could update the data in there too.</p>
  
  <p>My session IDs are MD5(fb_id + secret) so I could easily edit their session. The question is how can I detect if the session exists.</p>