private function _modified_headers($last_modified)
    {
        $modified_since = (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) ? stripslashes($_SERVER['HTTP_IF_MODIFIED_SINCE']) : FALSE;
        
        if (! $modified_since or $modified_since != $last_modified)
            return;
            
            // Nothing has changed since their last request - serve a 304 and exit
        header('HTTP/1.1 304 Not Modified');
        header('Connection: close');
        exit();
    }