[mgkimsal]

technically they'd just crash that one php process, not the whole web server. :)

I think the bigger 'why not' is ... 25 years of backwards compatibility.

[baby]

Oh right ^^ that’s actually one really cool thing about php web apps, no easy denial of service attacks

[mgkimsal]

(again) technically it could trigger a warning (actually, I thought perhaps it did that years ago??)

It's still pretty easy to DOS the server that is passing requests to PHP.

The 'shared nothing' approach has other benefits though - primarily not having to concern yourself with concurrency issues or shared memory resources. You can dive in to deep PHP and do that, but it's not something beginners need to be thinking about.