<?php
$db_username = '';
$db_password = '';
$db_name = '';
$db_host = 'localhost';
$db = new mysqli($db_host, $db_username, $db_password,$db_name);
if ($db->connect_error) {
    die('Error : ('. $db->connect_errno .') '. $db->connect_error);
}

?>
<form id="mc-form" class="mc-form" method="POST" action="#" novalidate="true">
<div class="newsletter-form">
<input autocomplete="off" required id="mc-email" placeholder="Email address" class="form-control" name="email" type="email">
<input class="mc-submit" type="submit" name="submit" value="Inschrijven">
<div class="clearfix"></div>
                                        <!-- mailchimp-alerts Start -->
<div class="mailchimp-alerts">
<div class="mailchimp-submitting"></div>
                                            <!-- mailchimp-submitting end -->
<div class="mailchimp-success"></div>
                                            <!-- mailchimp-success end -->
<div class="mailchimp-error"></div>
                                            <!-- mailchimp-error end -->
</div>
</div>
</form>
<?php

if (isset($_POST['submit'])) {
     $email = $_POST['email'];
        $sql = mysqli_query($db, "SELECT * FROM adressen WHERE email= '$email'  ");
        if (mysqli_num_rows($sql) != 0) { // kijken of de naam in DB betaat
            echo "Dit email adres staat al in onze database.";
        } else {
                mysqli_query($db, "INSERT INTO adressen (email) VALUES ('$email')");
                echo 'Gelukt!';
        }


}
?>