Posting XML From Js To Php

I have written a web page to create XML from user input. The user inputs a series of values, and these are turned into XML using javascript. The way I have done this is to load in a template XML file using an XMLHTTPrequest and append childNodes to it as required. Thus, what I end up with is a variable called "xmldoc" which is an XML document object. (If I do write(xmldoc) the answer I get is [object XMLDocument].)What I would like to do is to be able to save the thing as an xml file. To this end I have been trying to POST it to a php file that might do this, but to no avail; the php file does not seem to recognise it as an XML DOM object.