php 截取GBK文档某个位置开始的n个字符方法

#!/usr/bin/php<?phpdefine('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);?>