xmlhttprequest and the cache

Hello guys, I am writing a program using ajax (obviously). The javascript calls a php script which writes an xml file that the javascript then uses to generate stuff.

The problem comes from the fact that the xml file that I receive is out of date and in fact does not update unless I physically access the xml file and refresh the page. This leads me to believe that the old page is getting cached so the new file is not getting looked at.

So far I have tried
obj.setRequestHeader("Expires", "0");
obj.setRequestHeader("Cache-Control", "no-store");
obj.setRequestHeader("Cache-Control", "no-cache");
but to no effect. Any help would be appreciated.

SOLVED: setRequestHeader only works with POST not GET

update: setRequestHeader was not needed when using POST either