So I meant one thread per request at the php-fpm level, not Nginx/HTTPD level.
* Edit for spelling
Still, my understanding is that php-fpm spawns processes, not threads. What is "persistent" is the php-fpm daemon itself that's connected to nginx, but each request spawns a whole process which has to bootstrap your whole framework every time. That's the reason frameworks like symfony run almost mandatorily with something like opcache.
Coming from a Java background into PHP, this really confused me at first, until I started to think about PHP's approach as some kind of super-agressive garbage collection: everything gets released in that reqest thread once it's completed.
It's a fundemental design choice of the language that you have to embrace if you want to use PHP (I love it now, simplifies so much), or move on to another language.