reCAPTCHA v.3をプラグインなしで

//==========================================
//	recaptcha
//	================

if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['recaptcha_response'])) {

    // Build POST request:
    $recaptcha_url = 'https://www.google.com/recaptcha/api/siteverify';
    $recaptcha_secret = '〇〇〇〇〇〇〇〇〇〇〇〇'; // シークレットキー
    $recaptcha_response = $_POST['recaptcha_response'];

    // Make and decode POST request:
    $recaptcha = file_get_contents($recaptcha_url . '?secret=' . $recaptcha_secret . '&response=' . $recaptcha_response);
    $recaptcha = json_decode($recaptcha);

    // Take action based on the score returned:
    if ($recaptcha->score >= 0.5) { // botか人かの判定は中間の0.5と本メモでは設定
        // Verified - send email
    } else {
        // Not verified - show form error
    }

}

<!-- スラッグがcontactとentryのページに設置する場合 -->
<?php if ( is_page('contact') || is_page('entry') ) : ?>
	<script src="https://www.google.com/recaptcha/api.js?render=〇〇〇〇〇〇〇〇〇〇〇〇"></script>
	<script>
		grecaptcha.ready(function () {
			grecaptcha.execute('〇〇〇〇〇〇〇〇〇〇〇〇', {action: 'homepage'}).then(function(token) {
				var recaptchaResponse = document.getElementById('recaptchaResponse');
				recaptchaResponse.value = token;
			});
		});
	</script>
<?php endif; ?>

<input type="hidden" name="recaptchaResponse" id="recaptchaResponse">