public function _Login($username, $password=null){
        if(isset($username) && $username != null && $password == null){
            $username = $this->mysqli->escape_string($username);

            $getUser = $this->mysqli->query("SELECT * FROM user WHERE username = '".$username."' LIMIT 1");

            if($getUser->num_rows == 1){
                $this->smarty->assign('si', 'log_pw');
                $this->smarty->assign('userName', $username);
            } else {
                $this->smarty->assign('error', 'Username nicht vorhanden!');
                $this->smarty->assign('si', 'log');
            }

        } else {
            $this->smarty->assign('si', 'log_pw');
            $username = $this->mysqli->escape_string($username);
            $password = $this->mysqli->escape_string($password);
            $getUser = $this->mysqli->query("SELECT * FROM user WHERE username = '".$username."' LIMIT 1");
            if($getUser->num_rows == 1){
                $thisUser = $getUser->fetch_object();
                if($thisUser->password == md5($this->hash.$password.$this->hash)){
                    $_SESSION['username'] = $thisUser->username;
                    header("Location: index.php");
                    exit;
                } else {
                $this->smarty->assign('error', 'Passwort falsch!');
                $this->smarty->assign('si', 'log');
            }

            } else {
                $this->smarty->assign('error', 'Username nicht vorhanden!');
                $this->smarty->assign('si', 'log');
            }
        }
    }