Hmmm i would take a look at reactphp or if you want to implement it on your own you could check out https://pecl.php.net/package/libevent afaik there is another FPM like project https://github.com/PHPFastCGI/FastCGIDaemon
I would use generators as the main building blocks for the stream handling and with php 7.2 the streaming architecture of PHP gets another boost. generators will help you to keep your memory in check.
one of the questions is if you build it reactive or the classic while(true) loop + the threading system. to really scale independent you probably need a queue pipeline and maybe should be using an observer pattern to produce certain outputs.
I'm not even sure if i would go for the MVC pattern in this case, because you don't really need the controllers, but more of stateless service workers that pull the next state out of the queue and a separated state in form of session structure. the question is where to persist the state.
Since PHP usually is stateless (Request / Response) you should keep in mind what's running and how the current state of certain parts of the application are. hence the clear separation of state/structs to code logic (deterministic state machines)
I've built several such projects with distributed work systems the question is what is the main purpose ? replacing the classic PHP request response ?
It is fun to build such a thing I would first do the architecture design and separate the modules to smaller chunks by implementing an interface structure otherwise you probably will loose the oversight .... :)