php 截取GBK文档某个位置开始的n个字符方法

#!/usr/bin/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);

?>