php中计算中文字符串长度、截取中文字符串

/*  
  
* 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  
  
*/  
   
public function csubstr($str, $start=0, $length, $charset="utf-8", $suffix=true)   
   
{   
   
   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;   
   
}