Best way to build dynamic XML RSS feeds without adding much overhead.
 

What's the best way to build dynamic xml RSS feeds without adding much overhead to a database or server?

Example...
http://www.site.com/feeds/feed.php?aff=AffiliateName

And the feed displays all links attaching ?aff=AffiliateName to any URL in the feed.

So that bloggers, and such can just tack on their affiliate ID to the URL and get it inserted into the feed.

Obviously this can't be done adding echo statments in the XML file.

How are others doing this?