process.php

<?php

use React\ChildProcess\Process;
use React\EventLoop\Factory;
use function React\Promise\Stream\buffer;

require "vendor/autoload.php";

$proc = new Process("composer");

$proc->start($loop = Factory::create());


buffer($proc->stdout)->then(function ($bufferedOutput) {
    echo $bufferedOutput;
});

$loop->run();