秒速でPHPのフォームにreCAPTCHA v3を導入するサンプルコード

<?php
    // あらかじめ、recaptchaのサイト（https://www.google.com/recaptcha/intro/v3.html）で、サイトキーとシークレットキーを取得しておく。
    $recaptcha_site_key = "yoursitekey";
    $recaptcha_secret_key = "yoursecretkey";
    
    // recaptchaを表示する<FORM>〜</FORM>内に記述
    function recaptcha(){
        global $recaptcha_site_key;
        echo "<input type='hidden' name='recaptchaResponse' id='recaptchaResponse'><script src='https://www.google.com/recaptcha/api.js?render={$recaptcha_site_key}'></script><script>grecaptcha.ready(function(){grecaptcha.execute('{$recaptcha_site_key}', {action: 'sent'}).then(function(token){document.getElementById('recaptchaResponse').value = token;});});</script>";
    }
    // フォームが送信された後の処理 例：if(recaptcha_verify()){echo '認証成功';}else{echo '認証エラー';}
    function recaptcha_verify(){
        global $recaptcha_secret_key;
        $post_key = "recaptchaResponse";
        if(isset($_POST[$post_key]) && !empty($_POST[$post_key])){
            $response = file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$recaptcha_secret_key."&response=".$_POST[$post_key]);
            if(json_decode($response)->success){
                return true;
            }
        }
        return false;
    }
?>

<?php
if($_POST){
if(recaptcha_verify()){
echo '送信成功';exit;
}else{
echo '送信失敗';exit;
}
}
?>
<form method='post'>
<input type='text' name='name'>
<?php recaptcha()?>
<input type='submit' value='送信'>
</form>