Login/Security Problem

Hi there. I'm new to php and would like some insight on securing a website. Upon successful login to my site (checks against database for username and password) I assign a session variable called '$islogged' to 'yes'. On all other pages throughout my site I use the following code to determine if this variable is set, and if not redirect them to the login page. if($islogged = = "no"){ header("Location:index.php"); } elseif(EMPTY($islogged)) { header("Location:index.php"); } This seems to work, however, if I close out my browser and say type in main.php (this page has the above code) in the address bar I can still access the page. How can I fix this? Is there something else I could be doing to improve the functionality? Any insights would greatly be appreciated. Cheers Joe:)