php函数substr_replace中文乱码的替代解决方法

        $str = "中华人民共和国";
        $len = mb_strlen($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));