PHP comes from a time where bad code was less frowned upon, in general. This sort of thing was more normal back then.

In particular, this exact case you're talking about is being voted on right now, to promote it to an error instead in PHP 9.0: https://wiki.php.net/rfc/undefined_variable_error_promotion

It's looking like the vote will pass (as of writing, 33 to 8 ~= 80%, and 67% is needed).