PHP 中文无乱码截取

function substr_text($str, $start=0, $length, $charset="utf-8", $suffix=""){   if(function_exists("mb_substr")){      return mb_substr($str, $start, $length, $charset).$suffix;   }   elseif(function_exists('iconv_substr')){      return iconv_substr($str,$start,$length,$charset).$suffix;    }   $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));   return $slice.$suffix;}