Problems implementing encryption: mcrypt_cfb

Hey, thanks for looking. I would like to implement some sort of encryption to mask user id's. I looked at the example on the PHP website: http://au.php.net/manual/en/function.mcrypt-cfb.php This is exactly what I need. It does not quite work as the top post in that page suggests. Can anyone suggest how to fix this problem (and perhaps also post it under that section of the php function help) or suggest an alternate solution? Any help would be appreciated, Darren