Perhaps a more secure idea would be to require an opening stanza for the reverse - for content that should simply be printed to standard out? i.e. <?out to make content that should be outputed, not <?php for content that should be interpreted.

The easiest solution would be to define a new file extension/MIME type which is "PHP-by-default".

Unless you include it from somewhere in the web root, but that's the other insecure-by-default behaviour I was hinting at. With a secure-by-default web framework, it's not possible to get the code to show at all because it's not intermingled with the content.

Every block of PHP code must begin with '<?php', regardless of where it's located, or whether it's included from another file.

I do agree with you that this is a silly behaviour. But it's nothing to do with the web root.