Using get_browser() or $HTTP_USER_AGENT you could (neither are guaranteed, as it depends on suitable info from the browser) find out the platform the browser is running on. Whether it's enough is questionable, but it's all you've got to go on.
$useragent=get_browser();
$platform=$useragent['platform']; (eg, "WinCE")
With that, you can then redirect them using some suitable variation on header('Location:palmindex.php').