Using promise and concurrency in php to trigger multiple requests

concurrency.php

foreach ($listOfSites as $site) {
          file_put_contents('/tmp/lockprocess.lock','locked');

          $url = $site[0];
          // Add loop here to iterate if concurrency is bigger than a threshold.
          echo "starting ASYNC request for $url" . PHP_EOL;
          $promises[] = $client->requestAsync('GET', $url, [
            'headers' => [
              'User-Agent'   => 'AcquiaScrapperBot/0.1',
              'Accept'       => 'application/json',
              'X-WSF-Sesame' => 'k8nBKu7bK6rNZL3t'
            ],
            'http_errors' => false
          ])
            ->then(function($response) use ($url){
              echo PHP_EOL . 'querying:::' . $url . 'timestamp: ' . date("H:i:s");

              return array($url, $response->getStatusCode(), $response->getBody()->getContents()); //
            });
        }

        $results = Promise\unwrap($promises);

        foreach ($results as $result) {
          $siteCrawled = Array();
          $siteCrawled['url'] = $result[0];
          $siteCrawled['statusCode'] = $result[1];
          $siteCrawled['size'] = strlen($result[2]);
          $siteCrawled['footprint'] = md5($result[2]);

          $csvManager->writeCsvLine($siteCrawled,$fileToWrite);

    }