如何在smarty中调用递归函数？

$sql = "select menu_id , menu_name , parent_id from menu " ;
$dbc->setFetchMode(DB_FETCHMODE_ASSOC);
$res = $dbc->query($sql);
while($row = $res->fetchRow()){
    $menu[$row['parent_id']][$row['menu_id']] = $row['menu_name'];
}

function make_menu($parent)
{
    global $menu ;
    echo '<ul>';
    foreach($parent as $menu_id=>$menu_name)
    {
        echo '<li>'.$menu_name ; 
        if(isset($menu[$menu_id]))
        {
            make_menu($menu[$menu_id]) ;
        }
        echo '</li>';
    }
    echo '</ul>';
}
$P['menu_bilder_data'] = $menu[0] ; 
//menu :off
$smarty->register_function('make_menu' , 'make_menu') ;
$sql = "select menu_id , menu_name , parent_id