php 截取字符串

     /**
      * 方法库-截取字符串-【该函数作者未知】
      * @param string  $string 字符串
      * @param int     $length 字符长度
      * @param string  $dot    截取后是否添加...
      * @param string  $charset编码
      * @return string
      */
     public function cutstr($string, $length, $dot = ' ...', $charset = 'utf-8') {
         if (strlen($string) <= $length) {
             return $string;
         }
         $string = str_replace(array('&amp;', '&quot;', '&lt;', '&gt;'), array('&', '"', '<', '>'), $string);
         $strcut = '';
         if (strtolower($charset) == 'utf-8') {
             $n = $tn = $noc = 0;
             while ($n < strlen($string)) {
                 $t = ord($string[$n]);                //ASCIIֵ
                 if($t == 9 || $t == 10 || (32 <= $t && $t <= 126)) {
                     $tn = 1; $n++; $noc++;
                 } elseif (194 <= $t && $t <= 223) {
                     $tn = 2; $n += 2; $noc += 2;
                 } elseif (224 <= $t && $t < 239) {
                     $tn = 3; $n += 3; $noc += 2;
                 } elseif (240 <= $t && $t <= 247) {
                     $tn = 4; $n += 4; $noc += 2;
                 } elseif (248 <= $t && $t <= 251) {
                     $tn = 5; $n += 5; $noc += 2;
                 } elseif ($t == 252 || $t == 253) {
                     $tn = 6; $n += 6; $noc += 2;
                 } else {
                     $n++;
                 }
                 if($noc >= $length) {
                     break;
                 }
             }
             if ($noc > $length) {
                 $n -= $tn;
             }
             $strcut = substr($string, 0, $n);
         } else {
             for ($i = 0; $i < $length; $i++) {
                 $strcut .= ord($string[$i]) > 127 ? $string[$i].$string[++$i] : $string[$i];
             }
         }
         $strcut = str_replace(array('&', '"', '<', '>'), array('&amp;', '&quot;', '&lt;', '&gt;'), $strcut);
         return $strcut.$dot;
     }