reCAPTCHA PHP form handler

<?php
  $resp = $_POST['g-recaptcha-response'];
  $userIp = $_SERVER['REMOTE_ADDR'];
  $secretKey = 'YOUR_PRIVATE_KEY';
  $recaptchaUrl = 'https://www.google.com/recaptcha/api/siteverify'
                      . '?secret='   . $secretKey
                      . '&response=' . $resp
                      . '&remoteip=' . $userIp;

  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, $recaptchaUrl);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
  $json = curl_exec($ch);
  curl_close($ch);

  if (!$json) {
    // TODO: Throw error due to the server (HTTP 5xx)
  } else {
      $arr = json_decode($json, true);
      if(!$arr['success']) {
          // TODO: User identified as a machine, display an error on the form page
      } else {
          // TODO: User identified as a human, continue processing...
  }