PDO INSERT query after button push is not working

Question

I tried to get the user input from a form field after a button push and save them into a variable which I then used in the prepared statement to be inserted into the database. It doesn't work however. I checked the PHP manual but couldn't find any solution to my problem. Keep in mind: I'm new to OOP PHP with PDO. I tried to check whether I messed up with catching the values, I checked comma's, I checked the connection in the database file (the connection credentials are all ok), the name values in the index.php file. I really don't know why I am not seeing anything in the database.

Here's my question: Is there a syntactical error in my code?

<?php
require_once('database.class.php');

class queryClass {


public function insert() {

    $this->email = $_POST['email'];
    $this->wacht = $_POST['wacht'];
    $this->vn = $_POST['vn'];
    $this->an = $_POST['an'];

    if(isset($_POST['btn'])) {
    $query = "INSERT INTO test 
                    (emailadres, wachtwoord, voornaam, achternaam) 
              VALUES(:ID, :email, :wacht, :vn, :an)";

    $statement = $db->prepare($query);
    $statement->bindValue(':email', $this->email, PDO::PARAM_STR);
    $statement->bindValue(':wacht', $this->wacht, PDO::PARAM_STR);
    $statement->bindValue(':vn', $this->vn, PDO::PARAM_STR);
    $statement->bindValue(':an', $this->an, PDO::PARAM_STR);
    $statement->execute();
    $statement->closeCursor();

    }
    }
    }

?>