Recaptcha why doesn't this work?

Question

I'm trying to use the No captcha recaptcha but I have an issue. I use this code :

if(isset($_POST['g-recaptcha-response'])){
  $captcha=$_POST['g-recaptcha-response'];
}
if(!$captcha){
        $json['error'] = $this->language->get('error_captcha');
}

    if (!isset($json['error'])) {
        $this->model_catalog_review->addReview($this->request->get['product_id'], $this->request->post);

        $json['success'] = $this->language->get('text_success');
    }
}

but I always get the error message even if the captcha is filled out. How can I fix this?