Adding the script below at the top of each PHP page will prevent user from using the back button after logging out.
<?php
session_start();
// Check if session is active or not
if(!isset($_SESSION['session_variable_here'])) {
$_SESSION['session_variable_here']=NULL;
//Display message below if there is no more active session
echo "Sorry you are no longer active. Please <a href='index.php'>login</a> again.";
die();
}
else
//Your action here if session is active
}
?>
