Don't Forget to session_start()!

Are you trying to get to grips with how sessions work in PHP, but having trouble getting PHP to pick up the session you started in a previous request? If so, you may be the victim of unfortunate function naming.

You might (reasonably) assume from the name that you only need to call session_start() to create the session, but you actually need to use it to resume the session for the current request, too. From the PHP manual (emphasis mine):

session_start() creates a session or resumes the current one based on a session identifier passed via a GET or POST request, or passed via a cookie.

So, make sure to call session_start() whenever you want to do anything with the session, not only when you want to start a new one. (You could also set the session.auto_start setting to 1, which tells PHP to automatically start the session for every request.)