Yea as others have said, it's a shared nothing request model, but also single threaded, with built in memory and time limits, basically exactly how lambda runs (you can of course share memory, and multithread if you want to but its pretty uncommon practice in php).

The might be a nit, but this is more a limitation of how PHP has been used historically than the language itself.

I don't really see it as a "limitation" as it really makes grokking the code much easier than other languages. Probably >95% of libraries out there make this assumption as well, so its not just "how it is used" but also the culture and existing code that make this assumption.

That's an interesting way to put it

Probably the per-request nature of it when writing web apps.

It’s literally one execution per request (see php-fpm). It’s stateless-shared-nothing runtime by default.