Using wordpress functions in custom php script

Hello everyone. A brief description first:

I’m developing an as3 video player for a wp theme. In the theme, I’ve got a php script in a file that I use inside actionscript. This script generates a xml file with metadata from a particular post (custom fields, category, etc).

Inside the player, I’m trying to add html text, in particular html links to the Categories where the post belongs to (did u get that?? :S).

To get all the above, I retrieve the data directly from the database. But, as you may expect, i can’t get the categories links html from there. I know i can generate this by hardcoding the url with the catID (which I already got) inside actionscript. I wonder though what would happen if my client wants to use permalinks.

I assume that the “get_category_link( $category_id )” function can get the job done, regardless if wp is using permalinks or not. So this is the deal:

Since my script doesn’t interact at all with wordpress (i get everything from mysql), i can’t call any wordpress funtion inside my script. What files would i have to include inside my php in order to get access to wp functions? Or should I just create some vars and write configure this by hand on my script??