imap_setflag_full not setting \\Seen flag

I'm trying to use the imap_setflag_full function (PHP 3.0.15) to set the \\Seen flag for a message on a remote POP3 server: $status = imap_setflag_full($pop_stream, $msglist[$i], '\\Seen', ST_UID); The function returns true, indicating success. However, the flag isn't really being set. I can verify this by retrieving the message header with imap_header, and also because the call imap_search($pop_stream, "UNSEEN", SE_UID); subsequently returns the same message! Does anyone know what's going on here? Any help would be appreciated. Thanks, Chris