send_msg_batch.hh

async function send_msg_batch(Traversable<int> $targets, array $msg_body): Awaitable<void> {
  $chunks = new Vector(array_chunk($targets, 50));
  $to_process = $chunks->map($sub_targets ==> send_msg_batch_helper($sub_targets, $msg_body));
  await \HH\Asio\v($to_process);
}

async function send_msg_batch_helper(Traversable<int> $targets, array $msg_body): Awaitable<void> {
  $data = ['batch' => []];
  foreach ($targets as $target) {
    $data['batch'][] = [
      'method' => 'POST',
      'relative_url' => 'me/messages',
      'body' => http_build_query([
        'recipient' => ['id' => $target],
        'message' => $msg_body
      ])
    ];
  }

  $json_data = json_encode($data);
  $headers = [];
  $headers[] = 'Content-Type: application/json';
  $ch = curl_init('https://graph.facebook.com/v2.6' . "?access_token=" . $_ENV['CHAT_FB_TOKEN']);
  curl_setopt($ch, CURLOPT_POST,           TRUE);
  curl_setopt($ch, CURLOPT_POSTFIELDS,     $json_data);
  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
  await \HH\Asio\curl_exec($ch);
}