Alternatively, you could code the logic manually by increasing the id by one. For example, you're storing a first userID which normally starts as zero then increment it by one and finally store it in your table.
I just came accross this whilst looking for something else.
If I log into the database server on my free server (I use byethost) or locally on my dev copy, and click on the root link at the top, there's a tab labeled 'variables' And the first line is......(drum roll!)
'auto increment increment' just what you are looking for methinks!