header('Expires: ' . gmdate('D, d M Y H:i:s', $cachemtime + 3600) . ' GMT');
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', $mtime) . ' GMT');

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && 
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) === $mtime) {

   // This works around a bug in header(), where it would translate "HTTP/1.1
   // 304 Not Modified" to "Status: 304 OK".
   header((strpos(PHP_SAPI, 'cgi') === 0 ? 'Status: 304 Not Modified' : 
                                           'HTTP/1.1 304 Not Modified'));

} else {

   readfile($file);

}