There are several events
First of all, try to refresh the "backed" page as the page you backed to may be only the cached version from your browser.
If the problem is not this try:
setcookie("user");
or
setcookie("user","");
or even
setcookie ('user', '', (time () - 2592000), '/', '', 0);
Take a look at http://www.php.net/setcookie would helps!
Have a nice day!