php中英文字符截取

/*
 * 中文截取，支持gb2312,gbk,utf-8,big5
 * @param string $str 要截取的字串
 * @param int $start 截取起始位置
 * @param int $length 截取长度
 * @param string $charset utf-8¦gb2312¦gbk¦big5 编码
 * @param $suffix 是否加尾缀
 */
function csubstr($str, $length, $charset="", $start=0, $suffix=true) {
    if (empty($charset))
        $charset = "utf-8";

    if (function_exists("mb_substr")) {
        if (mb_strlen($str, $charset) <= $length)
            return $str;
        $slice = mb_substr($str, $start, $length, $charset);
    }
    else {
        $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]/";
        $re['big5'] = "/[\x01-\x7f]¦[\x81-\xfe]([\x40-\x7e]¦\xa1-\xfe])/";
        preg_match_all($re[$charset], $str, $match);
        if (count($match[0]) <= $length)
            return $str;
        $slice = join("", array_slice($match[0], $start, $length));
    }
    if ($suffix)
        return $slice . "…";
    return $slice;
}