php destroy session after some time
// Create a session variable called something like this after you start the session:
$_SESSION['user_start'] = time();
// Then when they get to submitting the payment, just check whether they're within the 5 minute window
if (time() - $_SESSION['user_start'] < 300) { // 300 seconds = 5 minutes
// they're within the 5 minutes so save the details to the database
} else {
// sorry, you're out of time
unset($_SESSION['user_start']); // and unset any other session vars for this task
}