Awesome! I was hoping to do this myself when I had a chance to learn MXML and Flex...
I only have one request, can you use the DOM API (php.net/dom) for common methods, i.e. "XML_MXML::load()" to load a file, "XML_MXML::loadXML()" to load a string.
I also think you should be using the DOM extension for this. As it requires PHP5 anyways, the DOM extension is enabled by default - make the most of it. In fact, if you *don't* use the DOM extension, I'm sure someone will request you make this a PHP4 package!
To surmise, a great start, it would benefit from using ext/DOM, thank you!