This PHP script will query your MySQL database's table where you keep your news, articles and such and basically create an XML file from that information. Once you look into the file itself, you can see that the query would need to be changed to suit your needs, but that's pretty much the end of it.
The script defines where the XML file is located that will get overridden in this case it would be .com/feeds/index.xml, but you can change that to what and where you want the file to be located. You would also need to change the permissions on that file so that the class could override the file.
The one thing that I actually do personally, is call the class itself once I get a result back from:
1. an article being added to the system.
2. an article being deleted from the system.
Download to get the PHP file to "Creating a Dynamic (xml) RSS Feed from PHP and MySQL"