Detect if PHP session exists
<p>Facebook now offer subscriptions to users so you can get realtime updates on changes. If my app recieves 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 id's are MD5(fb_id + secret) so I could easily edit their session. The question is how can I detect if the session exists.</p>
