Userid into an interval

[milla da killa]

Working on a code for a clan website which would assign each member a service number, even though this should be simple, I simply cannot get it to work. The plan was originally to take the user's id number, and simply add that to 0000 (thus getting a 4 digit number, such as 0123). Issue is, that I can't seem to get it convert to an integer so I can add it (and all attempts wind up echoing "4 + 0000" or something of the like). So, want to know how I could convert the user id to an integer so I can pull this off, or a code to get this to work.

Any help would be greatly appreciated.

/n00b question