php-recaptcha.php

<?php
if(isset($_POST['login'])) {
    if(!isset($_POST['g-recaptcha-response']) || empty($_POST['g-recaptcha-response'])) {
        echo 'reCAPTHCA verification failed, please try again.';
    } else {
        $secret = 'google_secret_key';

        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, 'https://www.google.com/recaptcha/api/siteverify?secret='.$secret.'&response='.$_POST['g-recaptcha-response']);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        $response = curl_exec($ch);
        curl_close($ch);
        $response = json_decode($response);

        if($response->success) {
            // What happens when the CAPTCHA was entered incorrectly
            echo 'Successful login.';
        } else {
            // Your code here to handle a successful verification
            echo 'reCAPTHCA verification failed, please try again.';
        }
    }
}