title: How to use amphp/parallel library for non-blocking process
body: <p>I want to use the amphp/parallel library for non-blocking process. I have a simple download file function which does a curl hit to the remote image file and save it to the local. I'm hitting this method through a REST API. Basically I want a process where aysnc download should be done on backend and it could be said as, REST API hit the function and function says "Hey, OK I'm downloading in background you can proceed ahead". Means non-blocking and API gets response as ok , not to wait. Meanwhile, if there is some network failure onto download, worker can restart the process in some time. How do I start?</p>
  
  <p>I have tried the following code, but did not work.</p>
  
  <pre><code>require_once "vendor/autoload.php";
  use Amp\Loop;
  use Amp\Parallel\Worker\CallableTask;
  use Amp\Parallel\Worker\DefaultWorkerFactory;
  
  \Amp\Loop::run(function () {
    $remote_file_url = "some remote image url"; //http://example.com/some.png
    $file_save_path = "save path for file"; //var/www/html/some.png
    $factory = new DefaultWorkerFactory();
    $worker = $factory-&gt;create();
    $result = yield $worker-&gt;enqueue(new CallableTask('downloadFile', [$remote_file_url, $file_save_path]));
  
    $code = yield $worker-&gt;shutdown();
  
    });
  </code></pre>
  
  <p>//downloadFile is a simple download function</p>
  
  <pre><code>function downloadFile($remoteFile, $localFile) {
      if (!$remoteFile || !$localFile) {
          return;
      }
      set_time_limit(0);
      $fp = fopen($localFile, 'w+');
      $ch = curl_init();
      curl_setopt($ch, CURLOPT_URL, $remoteFile);
      curl_setopt($ch, CURLOPT_TIMEOUT, 50);
      curl_setopt($ch, CURLOPT_FILE, $fp);
      curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
      $result = curl_exec($ch);
      curl_close($ch);
      fclose($fp);
  
      return $result ? true : false;
  }
  </code></pre>
  
  <p>I'm getting this error:</p>
  
  <pre><code>PHP Fatal error:  Uncaught Amp\\Parallel\\Worker\\TaskError: Uncaught Error in worker with message "Call to undefined function downloadFile()" and code "0" in /var/www/html/test/vendor/amphp/parallel/lib/Worker/Internal/TaskFailure.php:45\nStack trace:\n#0 /var/www/html/test/vendor/amphp/parallel/lib/Worker/TaskWorker.php(126): Amp\\Parallel\\Worker\\Internal\\TaskFailure-&gt;promise()\n#1 [internal function]: Amp\\Parallel\\Worker\\TaskWorker-&gt;Amp\\Parallel\\Worker\\{closure}()\n#2 /var/www/html/test/vendor/amphp/amp/lib/Coroutine.php(76): Generator-&gt;send(Object(Amp\\Parallel\\Worker\\Internal\\TaskFailure))\n#3 /var/www/html/test/vendor/amphp/amp/lib/Internal/Placeholder.php(130): Amp\\Coroutine-&gt;Amp\\{closure}(NULL, Object(Amp\\Parallel\\Worker\\Internal\\TaskFailure))\n#4 /var/www/html/test/vendor/amphp/amp/lib/Coroutine.php(81): Amp\\Coroutine-&gt;resolve(Object(Amp\\Parallel\\Worker\\Internal\\TaskFailure))\n#5 /var/www/html/test/vendor/amphp/amp/lib/Internal/Placeholder.php(130): Amp\\Coroutine-&gt;Amp\\{closure}(NULL, Object(Amp\\Parallel\\Worker\\Internal\\TaskFailur in /var/www/html/test/vendor/amphp/parallel/lib/Worker/Internal/TaskFailure.php on line 45
  </code></pre>
  
  <p>I have similar requirement as asked in <a href="https://stackoverflow.com/questions/44740557/how-does-amphp-work">How does amphp work</a> regarding the background running process.</p>