We had an "async PHP" discussion on Laravel #internals some weeks ago. The result of this discussion:
Even if PHP is evolving it is still not designed for async and it is better to use Node.JS for that case. Moreover, most of PHP built-in functions and libraries are sync. For example, you can't use async for PDO or file_get_contents(), however, you can use ReactPHP to improve a bit your general HTTP request-response logic and use Promises
gnugat.github.io/2016/04/13/super-speed-sf-react-…
https://www.youtube.com/watch?v=s6xrnYae1FU
And ReactPHP vs Node benchmark: https://gist.github.com/nkt/e49289321c744155484c
Don't use PHP for async, use Node.js. Use right tool for the right job.