怎么样使用PHP截取GBK文档某个位置开始的n个字符？

#!/usr/bin/php
<?php
define('INPUT_FILE', 't.txt');
define('OUTPUT_FILE', 'a.txt');
$pos = max(intval($argv[1]), 0); 
$len = max(intval($argv[2]), 0); 
$file_size = filesize(INPUT_FILE);
if($pos >= $file_size) exit;
$fp = fopen(INPUT_FILE, 'rb');
$point = 0; //current byte position
$string = ''; 
while(ftell($fp) < $file_size) {
 if($point >= $pos + $len) break;$byte = fread($fp, 1); 
 //php version >= 5.4
 $char = unpack('C', $byte)[1];
 if($char <= 0x7f) {
  //single byte
  if($point >= $pos) $string .= $byte;
  $point += 1;
  continue;
 } else {
  //double bytes
  if($point >= $pos) {
   $string .= $byte.fread($fp, 1); 
  } else {
   fseek($fp, 1, SEEK_CUR);
  }
  $point += 1;
  continue;
 } 
}
fclose($fp);
file_put_contents(OUTPUT_FILE, $string);
?>