Как отправить несколько запросов одновременно?

<?php 

  require __DIR__ . '/rolling-curl/src/RollingCurl/RollingCurl.php';
  require __DIR__ . '/rolling-curl/src/RollingCurl/Request.php';
  require __DIR__ . '/config.php';

  $stmgroups = $db->dbStream->prepare(" SELECT * FROM `groups_cover` WHERE `cover_status` = '1' AND `token_group` != '' AND `cover_morning_bg` != '' ORDER BY `date_render` ASC "); // LIMIT 10 
  try { $stmgroups->execute();} catch (PDOException $error) { trigger_error("Ошибка при подключении к базе: {$error}"); }

  $rows = $stmgroups->fetchAll(PDO::FETCH_ASSOC);

  foreach ($rows as $i => $d) {

 $rollingCurl = new \RollingCurl\RollingCurl();

$sites = array(
    'https://site.ru/render?group_id='.$d['group_id'].'&id_cover='.$d['id'] => array(
        CURLOPT_TIMEOUT => 3
    ),
);

}

foreach ($sites as $url => $options) {
    $request = new \RollingCurl\Request($url);
    $request->setOptions($options);
    $rollingCurl->add($request);
}

$rollingCurl
    ->setCallback(function(\RollingCurl\Request $request, \RollingCurl\RollingCurl $rollingCurl) {
        echo "Ok";
    })
    ->setSimultaneousLimit(3)
    ->execute();
?>

$sites = array(
    'https://site.ru/render?group_id='.$d['group_id'].'&id_cover='.$d['id'] => array(
        CURLOPT_TIMEOUT => 3 
    ),
);