On PHPMaster.com there's a tutorial posted that walks you through the creation of a podcast feed based around a Slim framework application and this skeleton application.

In this article, I’ll demonstrate how to use PHP to generate a podcast feed. We’ll create a simple administrative interface to configure the podcast metadata, add and list episodes, and then go through the generation of the podcast feed itself (which is simply an RSS document).

He walks you through the full process - setting up the application, creating the various management pages and making the associated views to add more information to the feed. He also includes the code/markup you'll need for adding episodes and generating the actual XML RSS feed output. You can download the full app if you want to see how it's all laid out.