[Note, this is a little more involved than some of the other GCI tasks]
The file includes/Feed.php in MediaWiki core is using php as a templating language.
For example, in RSSFeed::outHeader() it is directly outputting XML mixed with things like <?php print $this->getTitle(); ?>
It also seems to have inconsistent escaping (For example, calling wfExpandUrl() on a value already passed through htmlspecialchars(), where htmlspecialchars() is always supposed to be the last thing you do.
It would be nicer if this used [Mustache](https://en.wikipedia.org/wiki/Mustache_(template_system%29) and was consistently escaped using Mustache's double brace syntax.
See the folder includes/templates for example Mustache templates, and $templateParser->processTemplate() for examples of where Mustache is used in MediaWiki. And https://mustache.github.io/mustache.5.html