继承smarty类时遇到的有关问题，求指导！

继承smarty类时遇到的问题，求指导！！
我直接把smarty的libs目录拷进测试项目中，测试项目很简单，有两个文件demo.php和init.inc.php，然后我在init.inc.php做一些配置，比如界定符、模板目录之类，如果是以这种方式：

PHP code

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<?php 
    include_once './libs/Smarty.class.php';
    $tpl=new Smarty;
    $tpl->left_delimiter="<!--{";
    $tpl->right_delimiter="}-->";
?>

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PHP code

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<?php 
        require_once 'init.inc.php';
        $title="this is a title";
    $content="this is my content";
    $tpl->assign("title",$title);
    $tpl->assign("content",$content);
    $tpl->display("default/test.tpl");
?>

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PHP code

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       <?php 
    include_once './libs/Smarty.class.php';
    class mySmarty extends Smarty{
        function mySmarty(){
            $this->Smarty();
            $this->template_dir="./templates/";
            $this->compile_dir="./templates_c/";
            $this->config_dir="./configs/";
            $this->cache_dir="./cache/";
            $this->caching=true;
            $this->left_delimiter="<!--{";
            $this->right_delimiter="}-->";
            $this->assign('app_name','mySmartyTest');
        }
    }
       ?>

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PHP code

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<?php
    require_once 'init.inc.php';
    $tpl=new mySmarty;
    $title="this is a title";
    $content="this is my content";
    $tpl->assign("title",$title);
    $tpl->assign("content",$content);
    $tpl->display("default/test.tpl");
?>

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