Async HTTP integration with PHP

Async HTTP integration with PHP

As a way to integrate microservices I want to use HTTP calls and avoid the use of a MQ (Message Queue). 
If the app is small and contained in one host you could avoid the use of MQ.

Description of the scenario: 
I have one app (made with Symfony. Call it MainApp) and one microservice (made with Sylex. Call MicroApp), both served using PHP-FPM NGINX

  1) MainApp calls an URL POST /api/internal/do-something with Guzzle, 
  2) then MicroAPP do its work (it takes long) 
  3) MicroApp calls an URL from /api/internal/i-have-finished
  
To avoid keep MainApp waiting in 1). I've configured a very small guzzle timeout and it catches the exception. With this trick I have an Async HTTP call.
Are there any way to 2) in background without a MQ (and without anykind of worker proccess).

I'm tempted to use threads for 2) but I've read that they are only suitable for CLI (https://github.com/krakjoe/pthreads). 
I've heard that Spring have async REST.

What's the best way to solve this scenario and avoid waiting for 2) ? 

Silex after() isn't the solution but $app->finish() do the work pretty well