concurrency.php

<?php

use GuzzleHttp\Promise\EachPromise;
use GuzzleHttp\Psr7\Response;

$users = ['one', 'two', 'three'];

$promises = (function () use ($users) {
  foreach ($users as $user) {
    // don't forget using generator
    yield $this->getAsync('https://api.demo.com/v1/users?username=' . $user);		
  }
})();

$eachPromise = new EachPromise($promises, [
  // how many concurrency we are use
  'concurrency' => 4,
  'fulfilled' => function (Response $response) {
    if ($response->getStatusCode() == 200) {
      $user = json_decode($response->getBody(), true);
        // processing response of user here
      }
    },
  'rejected' => function ($reason) {
    // handle promise rejected here
  }
]);

$eachPromise->promise()->wait();