In a link from PHP Magazine:

"Seeing a mention of XML_XUL, the PEAR package, on a few blogs and in PHPMag, I thought it might be worth mentioning that I did evaluate using it for a project a while back. At that point, the code was not mature. However, It was already clear that the concept behind it is pretty flawed." - Alan Knowles.

He lists two ways that XML_XUL works: Read a XUL file, convert it into Big tree of XUL Elements, Manipulate it, then render it, and Start from scratch, building the whole tree manually PHP, and render. He also mentions some of the larger memory issues with XML_XUL such as performance issues, often needless, requiring the parsing of XML, and fatal flaw in that it totally obliterates the potential to WYSIWIG edit the application.

He goes on in his blog to wonder if, instead of using the XML_XUL system if the PHP DOM extension might not be a better, more powerful choice.