Google Recaptcha v3 example demo

Simple code to implement ReCaptcha v3

The basic JS code

<script src="https://www.google.com/recaptcha/api.js?render=your reCAPTCHA site key here"></script>
<script>
    grecaptcha.ready(function() {
    // do request for recaptcha token
    // response is promise with passed token
        grecaptcha.execute('your reCAPTCHA site key here', {action:'validate_captcha'})
                  .then(function(token) {
            // add token value to form
            document.getElementById('g-recaptcha-response').value = token;
        });
    });
</script>

The basic HTML code

<form id="form_id" method="post" action="your_action.php">
    <input type="hidden" id="g-recaptcha-response" name="g-recaptcha-response">
    <input type="hidden" name="action" value="validate_captcha">
    .... your fields
</form>

The basic PHP code

if (isset($_POST['g-recaptcha-response'])) {
    $captcha = $_POST['g-recaptcha-response'];
} else {
    $captcha = false;
}

if (!$captcha) {
    //Do something with error
} else {
    $secret="Your secret key here";
    $response = file_get_contents(
        "https://www.google.com/recaptcha/api/siteverify?secret=" . $secret . "&response=" . $captcha . "&remoteip=" . $_SERVER['REMOTE_ADDR']
    );
    // use json_decode to extract json response
    $response = json_decode($response);

    if ($response->success === false) {
        //Do something with error
    }
}

//... The Captcha is valid you can continue with the rest of your code
//... Add code to filter access using $response . score
if ($response->success==true && $response->score <= 0.5) {
    //Do something to denied access
}

You have to filter access using the value of $response.score. It can takes values from 0.0 to 1.0, where 1.0 means the best user interaction with your site and 0.0 the worst interaction (like a bot). You can see some examples of use in ReCaptcha documentation.