?php
//設置文件最長執行時間
set_time_limit(0);

if (isset($_GET['filename'])  !empty($_GET['filename'])) {
  $file_name = $_GET['filename'];
  $file = __DIR__ . '/assets/' . $file_name;
} else {
  echo 'what are your searching for?';
  exit();
}

if (file_exists($file)  is_file($file)) {
  $filesize = filesize($file);
  header('Content-Description: File Transfer');
  header('Content-Type: application/octet-stream');
  header('Content-Transfer-Encoding: binary');
  header('Accept-Ranges: bytes');
  header('Expires: 0');
  header('Cache-Control: must-revalidate');
  header('Pragma: public');
  header('Content-Length: ' . $filesize);
  header('Content-Disposition: attachment; filename=' . $file_name);

  // 打開文件
  $fp = fopen($file, 'rb');
  // 設置指針位置
  fseek($fp, 0);

  // 開啟緩沖區
  ob_start();
  // 分段讀取文件
  while (!feof($fp)) {
    $chunk_size = 1024 * 1024 * 2; // 2MB
    echo fread($fp, $chunk_size);
    ob_flush(); // 刷新PHP緩沖區到Web服務器
    flush(); // 刷新Web服務器緩沖區到瀏覽器
    sleep(1); // 每1秒 下載 2 MB
  }
  // 關閉緩沖區
  ob_end_clean();
  fclose($fp);
} else {
  echo 'file not exists or has been removed!';
}
exit();