php获得字符串内字符的个数(包含中文)

function getstrlen($str,$charset="utf-8") {
            $re['utf-8']   = "/[\x01-\x7f]|[\xc2-\xdf][\x80-\xbf]|[\xe0-\xef][\x80-\xbf]{2}|[\xf0-\xff][\x80-\xbf]{3}/";

            $re['gb2312'] = "/[\x01-\x7f]|[\xb0-\xf7][\xa0-\xfe]/";

            $re['gbk']    = "/[\x01-\x7f]|[\x81-\xfe][\x40-\xfe]/";

            $re['big5']   = "/[\x01-\x7f]|\x81-\xfe/";

            preg_match_all($re[$charset], $str, $match);

            return count($match[0]);
    }