This is something i had trouble with at first. I wanted to select the highest number (user_id) add 1 onto it (+1) and then insert it back into the database and then repeat.

The idea being i could just slap together a PHP file with a MYSQL connection and just repeat it. I did not want to be opening multiple connections and storing data in a variable so here is what i finally got:

INSERT INTO data (user_id) SELECT `user_id` + 1 FROM data ORDER BY `id` DESC LIMIT 1;

Again you may never need something like this but if you do, here it is.