Well, the simplest approach would be to treat your XML feed in the same way you treat HTML. You supply a URL to an XML page, and other servers hit it to get th news you're supplying. The fact that the XML page they're receiving is being dynamically generated by a PHP script shouldn't concern them in the least, and the fact that your PHP script is outputting XML instead of HTML shouldn't concern it.
A couple of caveats or two, though. First, if you've got short_tags on in your php.ini (so that <? can start a block of PHP) you'll have to put up with a bit of silliness to avoid problems with the <?xml tag at the top. Easier just to ditch short_tags.
Second, if you want the script to have an .xml file extension instead of .php, it's probably easier to have your server handle requests for .xml files by passing them to PHP (the way it already does for .php). Otherwise, either you will have to deal with headers that specify a different name or everyone else will have to.
Thirdly, a "Content-Type: text/xml" header would be a very good idea.
And of course, the people using your feed would have to know the structure of the document - designing that is your job!
