How do you intend to put PHP in an RSS document? If it serves an HTML one instead, then the RSS will no longer be available. You could try checking the HTTP headers to determine if the page is being fetched by an RSS reader or a browser, but such an approach is much more brittle than XSLT, which solves the problem exactly and easily. Not to mention it allows users to download browser extensions that override the provided formatting of XSLT documents with a custom standard one if they desire.
Not every application will set these correctly. It is less reliable than simply serving a static page. And that's the core of this issue. Before XSLT was removed, your website could be a directory of static content. You can put it in a zip file and send it anywhere you want. Now, even the most basic website (blog + feed) will require some dynamic content to work properly. We go from a world where static hosting is possible to one where it's less possible, and all because some browser implementors couldn't be bothered to upgrade a library to a safe version.
This would also break the workflow I have for my site, where I build it as a static directory locally during development and point Python's trivial HTTP server at it to access the content over localhost.
And it's totally insulting because the people removing this have created a (memory safe!) browser extension that lets you view XSLT documents, and put special logic in the browser to show users a message telling them to download that extension when an XSLT-styled document is loaded. They should bundle the extension with the browser instead of breaking my website and telling users where to fix it.
