Answer 1

use session_start() to start session and SID contains session address. Use that if user not log in session for that browser with user configuration will empty. Store your SID in database. But it will not prevent user to login to site from same computer and different browser. It is fixed for a particular browser not the whole computer. if you want to prevent login from other browser, use other methods. Session only stops further login from current logged browser. Learn how session works from PHP.net