PHP截取中文无乱码

这段代码好像是thinkphp里面的
/**
+----------------------------------------------------------
* 字符串截取，支持中文和其他编码
+----------------------------------------------------------
* @static
* @access public
+----------------------------------------------------------
* @param string $str 需要转换的字符串
* @param string $start 开始位置
* @param string $length 截取长度
* @param string $charset 编码格式
* @param string $suffix 截断显示字符
+----------------------------------------------------------
* @return string
+----------------------------------------------------------
*/

function msubstr($str, $start=0, $length, $charset="utf-8", $suffix=true)
{
if(function_exists("mb_substrs")){
$slice = mb_substr($str, $start, $length, $charset);
}elseif(function_exists('iconv_substr')) {
$slice = iconv_substr($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);
$slice = join("",array_slice($match[0], $start, $length));
}

if($suffix) return $slice."…";
return $slice;
}