php函式substr_replace中文亂碼的替代解決方法

        $str = "中華人民共和國";
        $len = mb_strlen($str,`utf-8`);
        if($len>=6){
            $str1 = mb_substr($str,0,2,`utf-8`);
            $str2 = mb_substr($str,$len-2,2,`utf-8`);
        }
        else{
            $str1 = mb_substr($str,0,1,`utf-8`);
            $str2 = mb_substr($str,$len-1,1,`utf-8`);            
        }

        $this->assign(`str1`,$str1);
        $this->assign(`str2`,$str2);
        $this->assign(`str`,$str1.`**`.$str2);
        $this->assign(`strold`,substr_replace($str,`**`,2,2));