public static function find_by_id($uid=0){
        // return an object populated based on user id
        global $database;
        $user = new User();

        $sql = "SELECT * FROM users ";
        $sql .= "WHERE  id=${uid} ";
        $sql .= "LIMIT 1";

        $result = $database->query($sql);

        if($database->num_rows($result)){
            $row = $database->fetch_assoc($result);
            $user->username = $row['username'];
            $user->password = $row['password'];
            $user->id = $uid;
        }

        return $user;

    }