if (isset($_POST['submit'])
    { // linje 5
        include 'db.inc.php';
        $username = $_POST['username'];
        $firstname = $_POST['firstname'];
        $lastname = $_POST['lastname'];
        $password = $_POST['password'];
        $query = mysql_query("INSERT INTO userinfo (userid, username, firstname, lastname, password) VALUES ('','$username', '$firstname', '$lastname', 'password')");
    }