php 截取GBK文档某个位置开始的n个字符方法

#!/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);
?>