ReactiveX/RxPHP

$source = \Rx\Observable::fromArray([1, 2, 3, 4]);

$source->subscribe(
    function ($x) {
        echo 'Next: ', $x, PHP_EOL;
    },
    function (Exception $ex) {
        echo 'Error: ', $ex->getMessage(), PHP_EOL;
    },
    function () {
        echo 'Completed', PHP_EOL;
    }
);

//Next: 1
//Next: 2
//Next: 3
//Next: 4
//Completed

$ git clone https://github.com/ReactiveX/RxPHP.git
$ cd RxPHP
$ composer install
$ php demo/interval/interval.php

$ composer require react/event-loop

$ composer require reactivex/rxphp

<?php

require_once __DIR__ . '/vendor/autoload.php';

use Rx\Observable;
use React\EventLoop\Factory;
use Rx\Scheduler;

$loop = Factory::create();

//You only need to set the default scheduler once
Scheduler::setDefaultFactory(function() use($loop){
    return new Scheduler\EventLoopScheduler($loop);
});

Observable::interval(1000)
    ->take(5)
    ->flatMap(function ($i) {
        return Observable::of($i + 1);
    })
    ->subscribe(function ($e) {
        echo $e, PHP_EOL;
    });

$loop->run();

Observable::interval(1000)
    ->flatMap(function ($i) {
        return Observable::fromPromise(\React\Promise\resolve(42 + $i));
    })
    ->subscribe(function ($v) {
        echo $v . PHP_EOL;
    });

$observable = Observable::interval(1000)
    ->take(10)
    ->toArray()
    ->map('json_encode');

$promise = $observable->toPromise();