PHP截取中文字符不乱码函数

function cn_substr_utf8($str, $length, $start=0){
        if(strlen($str) < $start+1){
                return '';
        }
        preg_match_all("/./su", $str, $ar);
        $str = '';
        $tstr = '';

        //为了兼容mysql4.1以下版本,与数据库varchar一致,这里使用按字节截取
        for($i=0; isset($ar[0][$i]); $i++){
                if(strlen($tstr) < $start){
                        $tstr .= $ar[0][$i];
                }else{
                        if(strlen($str) < $length + strlen($ar[0][$i]) ){
                                $str .= $ar[0][$i];
                        }else{

                                break;

                        }

                }

        }
        return $str;
}