<?php
if(isset($_POST['g-recaptcha-response']) && !empty($_POST['g-recaptcha-response'])):
    //your site secret key
    $secret = '9LuDh9kyetYYYYdT0jsVckScsH8Ks3KA';
    //get verify response data
    $verifyResponse = file_get_contents('https://www.google.com/recaptcha/api/siteverify?secret='.$secret.'&response='.$_POST['g-recaptcha-response']);
    $responseData = json_decode($verifyResponse);
    if($responseData->success):
        //Code if reCaptcha is verified
    endif;
endif;
?>