Session destroy on window close.

Hi Guys,

Any idea as to how we can destroy a PHP session on window close and not
only when the user clicks on logout link? There is a chance that the same
application can be used by different users with different
access privileges on the same computers. Right now m entirely depending on
$_SESSION variables to define the access privileges to each user, is this
the best method in terms of security?

Thanks,
Vinay Kannan.