Guzzle async http://docs.guzzlephp.org/en/latest/faq.html
sabre/event http://sabre.io/event/
ReactPHP http://reactphp.org/
Icicle.io https://github.com/icicleio/icicle
amphp https://github.com/amphp
jQuery deferred http://api.jquery.com/category/deferred-object/
Javascript promise https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise
You're missing the point of promises https://blog.domenic.me/youre-missing-the-point-of-promises/
Q https://github.com/kriskowal/q
Callbacks, Promises, Generators http://www.slideshare.net/wookieb/callbacks-promises-generators-asynchronous-javascript?related=1
httpbin https://httpbin.org/
What color is your function http://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/