reCAPTCHA v3 使い方（サンプル付き）

<script src='https://www.google.com/recaptcha/api.js?render=Site Key'></script>

<form id="test_form" method="POST" action="testAction">

    <!-- 適当な入力値 -->
    <input id="user_name" type="text" name="user_name" />

    <!-- 送信ボタン -->
    <input type="button" value="送信" onclick="commentSubmitFunc()"/>

</form>

<!-- reCAPCTHA -->

<script>

function commentSubmitFunc() {

    grecaptcha.ready(function() {

        grecaptcha.execute('Site Key', {action: 'Action Name'})

        .then(function(token) {

            $('#test_form').prepend('<input type="hidden" name="g-recaptcha-response" value="' + token + '">');

            $('#test_form').submit();

        });

    });

}</script>

$gRecaptchaResponse = $_POST["g-recaptcha-response"];

$reCaptchaUrl = "https://www.google.com/recaptcha/api/siteverify?secret=Secret key&response=" . $gRecaptchaResponse;

$response = @file_get_contents($reCaptchaUrl);

$response = json_decode($response,true);

if ($response['success'] != true or $response['score'] < 0.5 ){

    //エラー処理

}

{

  "success": true|false,      // whether this request was a valid reCAPTCHA token for your site

  "score": number             // the score for this request (0.0 - 1.0)

  "action": string            // the action name for this request (important to verify)

  "challenge_ts": timestamp,  // timestamp of the challenge load (ISO format yyyy-MM-dd'T'HH:mm:ssZZ)

  "hostname": string,         // the hostname of the site where the reCAPTCHA was solved

  "error-codes": [...]        // optional

}