RxPHP ( https://github.com/ReactiveX/RxPHP ) is a port of RxJS. We're going to be using Composer to handle all dependencies in our PHP projects. It has become a state of the art tool, so if you haven't used it before, download it first and check out some basic usage at  https://getcomposer.org/ .

Then, create a new directory and initialize a composer project:

$ mkdir rxphp_01
$ cd rxphp_01
$ php composer.phar init

Fill in the required fields by the interactive wizard and then add RxPHP as a dependency:

$ php composer.phar require reactivex/rxphp

When the library successfully downloads, composer will also create autoload.php file to handle all class auto-loading on demand.

Then, our code will print string lengths of different types of fruit:

// rxphp_01.php 
require __DIR__ . '/vendor/autoload.php'; 
 
$fruits = ['apple', 'banana', 'orange', 'raspberry']; 
$observer = new \Rx\Observer\CallbackObserver( 
    function($value) { 
        printf("%s\n", $value); 
    }, null, function() { 
        print("Complete\n"); 
    }); 
 
\Rx\Observable::fromArray($fruits) 
    ->map(function($value) { 
        return strlen($value); 
    }) 
    ->subscribe($observer); 

We first created an observer - CallbackObserver to be precise - which takes three functions as arguments. These are called on the next item in the stream, on error and when the input stream is complete and won't emit any more items.

The advantage of the CallbackObserver class is that we don't need to write a custom observer class every time we want to handle incoming items in some special and not very reusable way. With CallbackObserver, we can just write the callables for signals we want to handle.

When we run this example, we'll see:

$ php rxphp_01.php 
5
6
6
9
Complete

This example was very easy, but compared to the JavaScript environment, it's not very common to use asynchronous operations in PHP and, in case we do have to work asynchronously, it's probably something non-trivial. In Chapter 3, Writing a Reddit reader with RxPHP, we'll use Symfony Console component to handle all user input from the command line and, where we can, use similar principles to handling mouse events as we saw in the two RxJS examples above.

The JavaScript examples work very well as examples of what reactive programming using Reactive Extensions looks like and what its benefits are.