As the other comments say, it's not what the syntax is but the way it managed to get itself integrated into the webserver, originally via Apache mod_php. (mod_perl and even mod_tcl existed but were nowhere near as popular)

I believe mod_perl had the 'benefit' of persistent/shared state, which meant multiple users might conflict with each other (via shared memory, IIRC), and the mod_php - while it kept an interpreter bundled in each apache process - didn't leave anything around after a request.

Perl5 has HTML::Mason which runs on Apache mod_perl. The reason that never caught-on is the liability of running mod_perl compared with mod_php which is much better at isolating global variables.

I thought the comment was talking about PHP already being installed, so you could just start using it.

You can do it in a lot of templating engines like Jinja2 and ERB.

That's not comparable with PHP as ERB files are not executable by default within the document root.