function invisibleCaptcha($captcha) {
    $data = array(
        'secret'    => "SECRET",
        'response'  => $captcha
    );

    $verify = curl_init();
    curl_setopt($verify, CURLOPT_URL, "https://www.google.com/recaptcha/api/siteverify");
    curl_setopt($verify, CURLOPT_POST, true);
    curl_setopt($verify, CURLOPT_POSTFIELDS, http_build_query($data));
    curl_setopt($verify, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($verify, CURLOPT_RETURNTRANSFER, true);
    $response = curl_exec($verify);
    $response = json_decode($response);

    if($response->success === true){
        return true;
    }
    else{
        return false;
    }
}

if(invisibleCaptcha($_POST['captcha-response'])){
    // captcha onaylandı
}

<form method="post" action="">
<div class="form-group">
    <input type="text" name="name" class="form-control" />
</div>
<div class="form-group">
    <div class="g-recaptcha" data-sitekey="SITEKEY" data-badge="invisible" data-size="invisible" data-callback="setResponse"></div>
    <input type="hidden" id="captcha-response" name="captcha-response" />
</div>
</form>