Before you can store any information in session variables, you must first start up the session. To begin a new session, simply call the PHP session_start() function. It will create a new session and generate a unique session ID for the user.

Where do I put session start?

You want to put session_start(); at the top of your page before any other code. However, if you are using includes to make your life easier, it's best to put it at the very top of a file that is included in all files.

When should I start a session?

you can start session once user is authenticated. after that you can user related information in S_SESSION and access this info from anywhere. You should start session after verifying user's information, and than you can set user's uid to session variable.