parallel VS promise

You can leverage https://github.com/amphp/parallel or https://github.com/spatie/async without changing the app much.

it doesn't use fork to dispatch not anymore I suppose, when I last tried it that was the case. upon checking the code https://github.com/amphp/parallel/blob/master/lib/Worker/DefaultWorkerFactory.php they are now checking for various extensions like parallel or pthread and then defaulting to generators. so it uses generators now.

amphp (& amphp parallel ?)

Promises are a solution to asynchronicity in general. Promises are more of a specification, so anything claiming to implement Promises solves the same class of problem in the same way. Since php doesn't have language level wrappers for Await behavior it is going to look weird. Either you use something Object Oriented like Promises to handle the timing/async of your calls, or you use something functional like the pcntl functions to handle Process Control of multiple interdependent processes and the pipes/sockets between them. There are also separate Asnycronous libraries available that have a slightly different interface, but they probably just use promises in the backend. See https://github.com/reactphp/async and https://github.com/reactphp/promise for instance if you don't like curl.