Ok, ho fatto qusto e funziona ma non so se è corretto metologicamente.
il template l'ho cambiato così:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="it">
<head>
<title><tag:title /></title>
<link rel="stylesheet" type="text/css" href="template/css/style.css" />

</head>

<body>
<div id="container">
<div id="header">
<div id="testata"> </div>
</div>
<div id="top">


<tag:menuTop />
</div>
<div id="main">
<div id="leftbar">
<tag:menu />
</div>
<div id="content">
<tag:content />
</div>
<div id="footer">note di copyright</div>
</div>
</div>
</body>
</html>

mentre l'index,php cosi:
Codice PHP:
<?php
include_once('bTemplate.php');
$tpl = new bTemplate();

$title = 'flc';
$tpl->set('title', $title);


$tpl->set('menuTop', file_get_contents('menu.php'));
$tpl->set('menu', file_get_contents('left.php'));
$tpl->set('content', $tpl->fetch('main.php'));

echo $tpl->fetch('template/template.tpl');
?>