public function authorize($username, $password, $remember=false)
    {
        $query = "select id, username from users where
            username = :username and password = :password limit 1";
        $sth = $this->db->prepare($query);
        $salt = $this->getSalt($username);

        if (!$salt) {
            return false;
        }

        $hashes = $this->passwordHash($password, $salt);
        $sth->execute(
            array(
                ":username" => $username,
                ":password" => $hashes['hash'],
            )
        );
        $this->user = $sth->fetch();

        if (!$this->user) {
            $this->is_authorized = false;
        } else {
            $this->is_authorized = true;
            $this->user_id = $this->user['id'];
            $this->saveSession($remember);
        }

        return $this->is_authorized;
    }