[horde] I need a little help with _horde_hook_preauthenticate

[Demi]

Hello guys,
Got the calendar problem solved, thank for that, Chuck.
I like to use the auth. future "If this is not false, it is assumed to
be the URL of an alternate login screen which will be used in place of
horde's default login screen"
So I made following hook but I can't get it to work. Seems nothing
with sessions works so how would I do this?

I did this but it doesn't work

if (!function_exists('_horde_hook_preauthenticate')) {
     function _horde_hook_preauthenticate($userID) {
        $vdomain = preg_split('/\./', getenv('HTTP_HOST'), -1);
        $vdomain = array_reverse($vdomain);
        $vdomain = $vdomain[1] . "." . $vdomain[0];
        $userID = $_SESSION['auth']['user'];
        $userID = $userID . '@' . $vdomain;
        $userID = strtolower($userID);
        return $userID;
     }
}


Demi