Builder.com as posted a quick tutorial on working with the DOM functionality in PHP to create a dynamic XML document (Doctype and all).

Over the course of this article, I'll be introducing you to the main functions in this API, showing you how to programmatically generate a complete well-formed XML document from scratch and save it to disk.

They start off with creating the Doctype declaration before actually adding the content into the file (elements and text nodes), adding some attributes to the nodes, putting CDATA blocks inside them, and pushing out the results on the other side to be saved to a local "order.xml" file.

You can also download this article if you'd like to work with it (and its included PHP files) on your own.