IMAP: Marking a message unread/unseen?

I can't seem to find anything on php.net about how to mark a message as
unseen/unread on an IMAP server.

I'm thinking it has to do with imap_clearflag_full.

This is what I have and it's not working:

imap_clearflag_full($mbox,$key,'\\Seen',ST_UID);

$mbox = imap_open....
$key = message_uid

Any help?

-Charlie Davis