Generating user ids on a random forum website
Personally, when I need something to be secure and consistently unique is to add a secret to an incrementing ID, which I assume your DB already has.

Your members have ID's and they are incremental, but then you can design a secret phrase or some sort of algorithm to append to that number, and then mcrypt it, or, depending on how secure you need your UID, encrypt in other ways and then concatenate it to your desired length.