php中支持多种编码的中文字符串截取函数!

/*   
  * @todo 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, $start=0, $length, $charset="utf-8", $suffix=true)   
{   
 if(function_exists("mb_substr"))   
  return mb_substr($str, $start, $length, $charset);   
 $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;   
}