/**
 * 截取字符串
 *
 * @param  string $str           字符串
 * @param  int    $length        长度
 * @param  string $charset       编码
 * @param  bool   $suffix        是否加省略号
 * @return string
 */
function subs($str, $length, $charset="utf-8", $suffix=true) {
    $str = htmlspecialchars_decode($str);
	$start = 0;
	if (function_exists("mb_substr")) {
		if (mb_strlen($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;
}