recaptcha.php

<?php
$reCAPTCHA_site_key = reCAPTCHA_SITE_KEY; //assume define constant somewhere
$reCAPTCHA_secret_key = reCAPTCHA_SECRET_KEY; //assume define constant somewhere
if ($_POST['g-recaptcha-response']) {
    $recaptcha_response = $_POST['g-recaptcha-response'];
    $response = json_decode(file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=" . $reCAPTCHA_secret_key . "&response=" . $recaptcha_response . "&remoteip=" . $_SERVER['REMOTE_ADDR']));
    if (isset($response->success) && $response->success == 1) {
        //validate successfully.
    }
}