public static function checkCaptcha()
 {
     $userResponse = RequestHelper::getParameter("g-recaptcha-response");
     $url = ReCaptcha::CHECK_SERVER_URL;
     $data = array('secret' => ReCaptcha::SECRET_KEY, 'response' => $userResponse);
     $captchaServerResponse = ReCaptchaUtils::sendPostRequest($url, $data);
     return ReCaptchaUtils::getCaptchaCheckResult($captchaServerResponse);
 }
<?php

require_once __DIR__ . '/wp-load.php';
require_once ABSPATH . WPINC . '/lib/utils/class-email-utils.php';
require_once ABSPATH . WPINC . '/lib/utils/class-recaptcha-utils.php';
$checkMessageDataResult = EmailUtils::checkMessageData();
if ($checkMessageDataResult != EmailUtils::ERR_SUCCESS) {
    returnFromScript($checkMessageDataResult);
}
if (!ReCaptchaUtils::checkCaptcha()) {
    returnFromScript(ReCaptchaUtils::ERR_BOT_CHECK_FAILED);
}
$sendMessageResult = EmailUtils::sendMessage();
returnFromScript($sendMessageResult);
function returnFromScript($response)
{
    exit($response);
}