I have added a zip_cd field to my mysql userdata table and would like to ask new users their zip code when they register. I went to /includes/register.inc.php and made that edit okay. (To make it easy I am just making it optional).

I am doing this more to learn than for any other reason, and figure this should be a really simple beginner project. However, I would just be stabbing in the dark as to what other directory/file(s) I will need to modify.

Can anyone tell me what other files?

ps. If you prefer to tell me to keep looking and I'll eventually figure it out, that's fair, too, but there are many files to search through.