Google ReCaptcha secure token generation using slushie class from: https://github.com/slushie/recaptcha-secure-token/blob/master/lib/ReCaptchaToken.php Secure token makes it possible to use ReCaptcha for testing on localhost.

recaptcha_secure_token_generation.php

<?PHP
use ReCaptchaSecureToken\ReCaptchaToken as ReCaptchaToken;
require_once("ReCaptchaToken.php");

//Generate recaptcha token
$recaptcha_config = [	
        'site_key' 		=> 'your-site-key', 
				 'site_secret' 	=> 'your-site-secret-key'
			];
$recaptcha_token = new ReCaptchaToken($recaptcha_config);
$recaptcha_session_id = uniqid('recaptcha');
$recaptcha_secure_token = $recaptcha_token->secureToken($recaptcha_session_id);

//template file with the recaptcha
<div class="g-recaptcha" data-sitekey="<?= $recaptcha_config['site_key'] ?>" 
data-stoken="<?=  $recaptcha_secure_token ?>" ></div>