On css-tricks David Khourshid wrote a good introduction to RxJS.

According to ReactiveX.io: "ReactiveX is a library for composing asynchronous and event-based programs by using observable sequences." That's a lot to digest in a single sentence. In this article, we're going to take a different approach to learning about RxJS (the JavaScript implementation of ReactiveX) and Observables, by creating reactive animations.

https://css-tricks.com/animated-intro-rxjs/

If you want to toy around with the same ideas in PHP check out RxPHP