问谷歌reCaptcha回应“未明(承诺)为空”

EN

<div class="text-xs-center" style="text-align: center; height:150px;">
    <p style="color: black;"> Complete the verification: </p>
    <div style="display: inline-block;" class="g-recaptcha" data-sitekey="xxxxxxxxxxx" data-callback="callback"></div>
</div>

function callback() {
    if (grecaptcha === undefined) {
        alert('Recaptcha non definito'); 
        return; 
    }

    var response = grecaptcha.getResponse();
    console.log(response);

    if (!response) {
        alert('Coud not get recaptcha response'); 
        return; 
    }

    $.ajax({
    'url' : 'validate-recaptcha.php',
    'type' : 'POST',
    'data' : {
        'response' : response   
    },
    'success' : function(data) {              
        alert('Data: '+data);
    },
    'error' : function(request,error)
    {
        alert("Request: "+JSON.stringify(request));
    }
    });
    grecaptcha.reset();
}

<?php
//debug
$fp = fopen('debug.txt', 'a');
fwrite($fp, print_r($_POST, TRUE));
fclose($fp);
//enddebug

if (empty($_POST['recaptcha'])) {
    exit('Please set recaptcha variable');
}
// validate recaptcha
$response = $_POST['recaptcha'];
$post = http_build_query(
    array (
        'response' => $response,
        'secret' => 'yoursecretkey',
        'remoteip' => $_SERVER['REMOTE_ADDR']
    )
);
$opts = array('http' => 
    array (
        'method' => 'POST',
        'header' => 'application/x-www-form-urlencoded',
        'content' => $post
    )
);
$context = stream_context_create($opts);
$serverResponse = @file_get_contents('https://www.google.com/recaptcha/api/siteverify', false, $context);
if (!$serverResponse) {
    exit('Failed to validate Recaptcha');
}
$result = json_decode($serverResponse);
if (!$result -> success) {
    exit('Invalid Recaptcha');
}
exit('Recaptcha Validated');