Auto Increment Problem

[makamo66]

I need to create a tiles table that has a structure like this for http://www.myownmealplanner.com:

id user_id sub_tile_id
1	1	1
2	1	2
3	1	3
4	2	1
5	2	2
6	2	3
7	3	1
8	3	2
9	3	3

etc.
I can't just create new tables for new users because I'm using cakephp and that would require new models, views, and controllers for every new table. How do I get the sub_tile_id to auto_increment starting at every new user id?