Easy RSS Generator is a set of PHP classes that make your life easier when generating an RSS feed. You can output the generated XML to a file, or just return it.
It’s so simple to use, and comes with great documentation.
Take a look at http://z.numbface.net/documentation/ which gives you a great overview and tutorial on how to use it easily.
This uses RSS 2 .0. You can read the specification at http://www.rssboard.org/rss-profile