PHP Verification Function for Google recaptcha Server side code

php-google-recaptcha-verification-function.php

<?php
/*
 * Verifies if the captcha data via Google and redirects to the form page if the captcha does not pass.
 * 
 * Parameters:
 * Gcaptcha response - the form data submitted with the request.
 ** Example call if your form used method='get': verify_captcha($_GET['g-recaptcha-response'], "Secret key here");
 ** Example call if your form used method='post': verify_captcha($_POST['g-recaptcha-response'], "Secret key here");
 */
function verify_google_captcha($gcaptcha_response, $secret) {
  //open a new curl connection
  $ch = curl_init();
  $url = "https://www.google.com/recaptcha/api/siteverify";

  $fields = array(
    "secret" => $secret,
    "response" => $gcaptcha_response,
  );

  $fields_string = "";
  foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
  rtrim($fields_string, '&');

  //set the url, number of POST vars, POST data
  curl_setopt($ch,CURLOPT_URL, $url);
  curl_setopt($ch,CURLOPT_POST, count($fields));
  curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

  //execute post request
  $result = curl_exec($ch);
  //close connection
  curl_close($ch);

  $json_data = json_decode($result);

  //If the captcha was declined, redirect the user to the same page with the query var "captcha" set to "not-entered"
  if($json_data->success == false) {
    $redirect_to = $_SERVER["HTTP_REFERER"];
    if(!strpos($redirect_to, "?")):
      $redirect_to.="?captcha=not-entered";
    else:
      $redirect_to.="&captcha=not-entered";
    endif;
    header("Location: " . $redirect_to);
    exit;
  }
}
?>

This works well, thanks!