To authenticate using kerberos V / GSSAPI, you might need to add "user=" to the connection string.. eg:
$mbox = imap_open( "\{imap.example.com:143/imap/notls/user=" . $user . "}INBOX", $user, $passwd );
Our IMAP servers won't allow a user other than the user specified in the kerberos credentials connect using those credentials unless you specify that extra "user=" in the connection string. Passing it as an argument to imap_open() doesn't seem to be enough.