The best part about XML is its required well-formed nature. Worst comes to worst, you can create you're own XML handling. I have done this once on PHP4, but as soon as I could use (I think it was) the DOMXML library I dumped the ones I created. Why reinvent the wheel.
I don't have this code anymore, but I recall it being simple pattern matching. It did require a full load of the file so it wouldn't work if the file was too large to handle (at the time, about 64mb in size).
As Indegoesdr mentioned, there are a lot of XML handling functions in PHP.
If you're on PHP4, you'll be looking at DOMXML - you'll need the libxml library if I'm not mistaken to use it. If you're on PHP5, look into the DOM - simplexml can be converted to and from DOM if I'm not mistaken. DOM is also enabled by default in PHP5, but can also be disabled.
header('HTTP/1.1 420 Enhance Your Calm');