reCAPTCHA v2の使い方（サンプル付き）

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

<form ...>

        <div class="g-recaptcha" data-sitekey="*******************" data-callback="enableBtn"></div>

        <input name="btn" 
               type="submit" 
               value="button"
               disabled/>

</form>

<script type="text/javascript">
function enableBtn(code) {
        if(code !== ""){
                $(':submit[name=comment_btn]').removeAttr("disabled");
        }
}
</script>

<?php
$gRecaptchaResponse = $_POST["g-recaptcha-response"];
$reCaptchaSecretKey = "..."; 
$reCaptchaUrl = "https://www.google.com/recaptcha/api/siteverify?secret=".$reCaptchaSecretKey."&response=" . $gRecaptchaResponse;
$response = @file_get_contents($reCaptchaUrl);
$response = json_decode($response,true);

//var_dump($response);

if ($response['success'] == true){
    print "成功";
} else {
    print "失敗";
}
 
?>