PHP doesn't get to decide what happens for each new connection. That's determined upstream, and there's a lot of different pieces of software people choose to do that. Could be directly a webserver like apache's mod_php, or something like fastcgi, php-fpm, etc.

All of those front-ends chose to implement a model of a php process pool. Multiple processes, each with a single php interpreter running. Incoming connections are sent to a process in the pool. So, if the pool is 5 processes, and you get a 6th concurrent connection, that one waits.

However, you can write a C program with a single process and distinct php interpreter per thread. One example: https://github.com/basvanbeek/embed2-sapi

To date, nobody has taken something like that and done the work to connect it to a front end handler.