There is zero arguments for having PHP bundled as default with the OS. It always ends up being an old PHP version, crippled with bugs, and now apple has one more thing to worry about in the update release cycle.
If someone really wants to still keep on using PHP its just a brew install away.
Suggesting "just" brew install php as in original post is silly to me.
You have to install homebrew first, which is a non-trivial task requiring root access.
It should be trivial to anyone needing it, and by extension anyone needing tools you usually install with homebrew. Unless this is the case you should not need PHP in the first place.
If someone really wants to still keep on using PHP its just a brew install away.