You could try adding some headers to force browser to fetch... along the lines of:
<?php
if(!session_id()) {
@session_start();
}
header('Expires: Sat, 26 Jul 1997 05:00:00 GMT');
header('Last-Modified: ' . gmdate( 'D, d M Y H:i:s') . ' GMT');
header('Cache-Control: no-store, no-cache, must-revalidate');
header('Cache-Control: post-check=0, pre-check=0', false);
header('Pragma: no-cache');
if(isset($_SESSION['form_submitted'])) {
unset($_SESSION['form_submitted']);
header('Location: ?' . uniqid());
#header('Refresh: 0');
}
?>