gistfile1.php

<?php

require "vendor/autoload.php";

$loop = \React\EventLoop\Factory::create();
$filesystem = \React\Filesystem\Filesystem::create($loop);

$filesystem
    ->file('nonexisting.txt')
    ->exists()
    ->then(function(){
        var_dump('EXISTS');
        die();
    })
    ->otherwise(function(\Exception $exception) {
        var_dump('Does not exist');
        die();
    });

$loop->run();