php 中文截取函数

/*
 * 中文字符截取
 * @param   string   $string        被处理的字符串
 * @param   int      $start         开始截取的位置
 * @param   int      $length        截取的字符长度
 * @param   string   $charset       字符编码
 * @param   string   $dot           缩略符号 * @return string
 */
  function cutstr($string, $start, $length, $charset = "utf-8", $dot = '') { 
   if(function_exists("mb_substr")){
     if(mb_strlen($string,$charset)>$length){
       return mb_substr($string,$start,$length,$charset).$dot;
     }
     return mb_substr($string,$start,$length,$charset);
   }
   $new = '';
   if($charset==='gbk'){
     $string = iconv("gbk","utf-8",$string);
   }
   $str = preg_split('//u',trim($string));
   for($i = $start,$len = 1;$i<count($str)-1 && $len<=$length;$i++,$len++){
     $new .= $str[$i+1];
   }
   if($charset==='gbk'){
     $new = iconv("utf-8","gbk",$new);
   }
   return count($str)-2<$length?$new:$new.$dot;
 }
 echo cutstr('暗色调adsa暗色调',0,1,'utf-8','....');