small php script

Hi, I have a form on three post pages (under three different categories), and I would like the correct category to be set and submitted according to the category the user is currently under. (in other words they don't manually choose the category, its auto-detected.) For example; user is in the "makeup tips" category, so their submitted post is filed under that category. The category doesn't need to be visible in the user form, I imagine it would be a simple php script to detect the current category id and ensure that the post is submitted with that id. I'm using the "user submitted posts" plugin, I can provide an FTP login and Admin access, the blog is not yet live.