RSS Aggregator - Pull from multiple sourcesand combine into 1 feed output

I would like an app (already made preferrably) that will pull multiple items (or multiple RSS feeds) into the system and combine them into one output feed. You might call this an RSS Feed Constructor. It will probably have to be server based using PHP and MYSQL but if you're already got something written in other languages or format I'd be interested as well. My goal for this project: Be able to input information namely multiple RSS feeds, plug them into the system and the script to spit out a properly formed combined RSS feed with those items in it. If you have any questions please feel free to ask! If you have a system that can pull from database information that would be even more usefull and helpful for my uses! Thanks!