The PHP.sublime-syntax will work really well for this.

Set your XML file to PHP to test it out. If you want to make it permanent for all XMLs:

You can duplicate the content of that sublime-syntax file into a new sublime-syntax file, and change the file extensions listed in the file from PHP to XML. Then, when viewing one of your XML files, on the file type menu, set your file (and all files like it) to always use your new one.