I'm creating a Wordpress plugin and, being a newbie in the development on this platform, I'm stuck on this problem.

I'd like to have posts in the loop filtered by categories, defined by the user through an admin page. I would actually like to be able to modify query_post() parameters in the plugin, but the only trick I found is to re-run the query_post() with my user-defined criteria, thing that I would like to avoid.

I'm sure the solution is evident, but can't find it!