Thanks Old Pendant. Oh, I think you misunderstood the second question quite alot. When I said "users" I meant MySQL database users!
I get the idea that what I was thinking of accomplishing can be done better by a different approach. I was thinking of changing users (permissions) if a table didn't exists, so that I could create that table. In the case of a new forum section being added to a list of forum sections — which I would regularly only need to view — I could then add a table if it doesn't have a table for that section. But it makes more sense to create that table at the same time as adding a new section to the list of sections, which would probably be done in an admin panel.
Maybe I just have to structure it right.