Strange thing with Google nocaptcha - php

Question

I have a strange problem with Google Captcha. I've tried all kinds of php codes from different tutorials, but the result is exactly the same every time...
The problem is this:
it shows up correctly
if you check the box, it works correctly
if you then send the form it works correctly
but... if you don't check the box, the form is still sent!
So, in other words, it's only on the form as a decorative piece. What could be the problem? It's probably something very simple, but I'm totally missing it.
Help or insights are very much appreciated! Thanks in advance!
Addendum
The following is the code that came with the template I used:
require_once('recaptcha-php-1.11/recaptchalib.php');
if ($use_captcha == 1) {
$resp = null;
$error = null;
$reCaptcha = new ReCaptcha($secret);
$secret = "MY SECRET KEY HERE";
$captcha_error_message = '<div class="pi-alert-danger fade in"><button type="button" class="pi-close" data-dismiss="alert"><i class="icon-cancel"></i></button><p>Bewijs dat je geen robot bent!</p></div>';
if (isset($_POST["captcha_response"]) && $_POST["captcha_response"] != '') {
$resp = $reCaptcha->verifyResponse(
$_SERVER["REMOTE_ADDR"],
$_POST["captcha_response"]
);
if ($resp && $resp->success != true) {
echo $captcha_error_message;
exit();
}
} else {
echo $captcha_error_message;
exit();
}
}

Answer 1

You have to check if the captcha was solved (at your PHP-Script which do anything with the Form-data)
Like this:
function checkCaptcha($recaptchaResponse) {
$recaptchaPrivateKey = 'Your Private Key';
if(! $recaptchaResponse)
return false;
$recaptchaObj = new ReCaptcha($recaptchaPrivateKey);
$response = $recaptchaObj->verifyResponse($_SERVER["REMOTE_ADDR"], $recaptchaResponse);
if($response != null && $response->success)
return true;
return false;
}
If you don't include a function like this to your form function, your server will say, the form is okay, because it don't know about the captcha.
Note, that you have to include the Google-captcha Libarys-File as well. You can find it here:
https://github.com/google/recaptcha/blob/1.0.0/php/recaptchalib.php (Worked for NoCaptcha as well)