Open issue in Guzzle 6

ApiTest.php

<?php

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

use GuzzleHttp\Client;
use Psr\Http\Message\StreamInterface;
use Psr\Http\Message\ResponseInterface;
use GuzzleHttp\Exception\RequestException;

use GuzzleHttp\Pool;
use GuzzleHttp\Psr7\Request;
use GuzzleHttp\Promise;

class ApiTest {
    public $client;
    public static $instance;
    
    public function __construct() {
        $this->client = new Client([
            'base_uri' => '',
            'headers' => [
                'Accept'        => 'application/json',
                'Content-Type'  => 'application/json'
            ],
            'verify'            => false,
            'http_errors'       => false
        ]);
        self::$instance = $this;
    }

    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
    public function bulkRequest($params,&$res,$concurrency=10,$fulfillCallback=null,$rejectCallback=null){
        if(empty($params)) die('Parameters must array.');
        if(empty($fulfillCallback)){
            $fulfillCallback = function(ResponseInterface $response, $index) use (&$res) {
                array_push($res,$response);
            };
        }

        if(empty($rejectCallback)){
            $rejectCallback = function($reason, $index) use (&$res) {
                array_push($res,$reason);
            };
        }
        
        $requests = function ($params) {
            foreach ($params as $v) {
                yield new Request($v['method'],$v['url'],$v['headers'],$v['body']);
            }
        };

        (new Pool(
            $this->client,
            $requests($params),[
                'concurrency' => $concurrency,
                'fulfilled' => $fulfillCallback,
                'rejected' => $rejectCallback
            ]
        ))->promise()->wait();
    }

}

test.php

<?php
require_once 'ApiTest.php';
$client = ApiTest::getInstance();

$interval = 100;
$concurrency = 10;
$res = array();

echo sprintf("Printing memory usage every %d requests\n", $interval);
echo "Fetching package list... ";
$packageNames = json_decode($client->client->get('https://packagist.org/packages/list.json')->getBody()->getContents())->packageNames;
echo 'done. (' . count($packageNames) . " packages)\n\n";

$params = function($packageNames){
    foreach($packageNames as $v){
        yield array(
            'method' => 'GET',
            'url' => "https://packagist.org/p/{$v}.json",
            'headers' => [],
            'body' => null
        );
    }
};

$fulfillCallback = function(\Psr\Http\Message\ResponseInterface $response, $index) use (&$counter, $interval) {
    $counter++;
    if ($counter % $interval === 0) {
        echo sprintf(
            "Processed %s requests. Memory used: %s MB\n",
            number_format($counter),
            number_format(memory_get_peak_usage()/1024/1024, 3)
        );
    }
};

$rejectCallback = function($reason, $index) use (&$counter, $interval) {
    $counter++;
    if ($counter % $interval === 0) {
        echo sprintf(
            'Processed %s requests. Memory used: %s MB',
            number_format($counter),
            number_format(memory_get_peak_usage()/1024/1024, 3)
        );
    }
};

$client->bulkRequest($params($packageNames),$res,$concurrency,$fulfillCallback,$rejectCallback);