尝试将数据插入数据库PHP PDO

Question

I've tried everything and just cant seem to get this working.. it's probably a silly mistake I can't see but any help is appreciated.

As stated in the question I'm trying to insert records into a table via a form. I have a functions.php which includes my database.php with the pdo connection (all working fine) class with the following function in it:

function insertStaffUser($username, $password, $role) {

    include('database.php');
    try {
        $query = "INSERT INTO users (userid, username, password, role) VALUES (default, :username, :password, :role)";
        $stmt->$db->prepare($query);
        $stmt->bindParam(':username', $username);
        $stmt->bindParam(':password', $password);
        $stmt->bindParam(':role', $role);

        $result = $stmt->execute();
        if($result) {
            echo "INSERTED SUCCESSFULLY";
        } else {
            echo "error inserting";
        }
    } catch(PDOException $e) {
        echo "Error: " . $e->getMessage();
    }
}

And the following code is the one in my html class which is addUser.php with 3 text fields (new username, password and role).

<?php
if(isset($_POST['submit'])) {
    $new_username = $_POST['username'];
    $new_pass = $_POST['password'];
    $new_role = $_POST['role'];
    insertStaffUser($new_username, $new_pass, $new_role);
}
?>

Can anyone see what's wrong with this or what I'm doing wrong, thanks for the help!