PHP
(quick guide & example)
For local testing, also register "localhost", "127.0.0.1", or "::1".
reCaptcha will provide 2 keys - Site key and secret key.
HTML CONTACT FORM <form method="post"> <input type="email" name="email"> <textarea name="msg"></textarea>
<div class="g-recaptcha" data-sitekey="SITE-KEY"> </div> <input type="submit" value="Go!"> </form>
LOAD RECAPTCHA LIBRARY <script src="https://www.google.com/ recaptcha/api.js"></script>
SECRET KEY IN RECAPTCHA URL $url = "https://www.google.com/ recaptcha/api/siteverify?secret= SECRET-KEY&response=". $_POST["g-recaptcha-response"];
RECAPTCHA VERIFY $verify = json_decode (file_get_contents($url));
PROCEED IF OK if ($verify->success) { $to = "jon@doe.com"; $sub = "CONTACT"; $body = ""; foreach ($_POST as $k=>$v) { $body .= "$k: $v\r\n"; } mail($to, $sub, $body); } else { echo "Invalid captcha"; }