/**
   * Add files and sub-directories in a folder to zip file.
   * @param string $folder
   * @param ZipArchive $zipFile
   * @param int $exclusiveLength Number of text to be exclusived from the file path.
   */
  private function folderToZip($folder, &$zipFile, $exclusiveLength) {
    $handle = opendir($folder);
    while (false !== $f = readdir($handle)) {
      if ($f != '.' && $f != '..') {
        $filePath = "$folder/$f";
        // Remove prefix from file path before add to zip.
        $localPath = substr($filePath, $exclusiveLength);
        if (is_file($filePath)) {
          $zipFile->addFile($filePath, $localPath);
        } elseif (is_dir($filePath)) {
          // Add sub-directory.
          $zipFile->addEmptyDir($localPath);
          $this->folderToZip($filePath, $zipFile, $exclusiveLength);
        }
      }
    }
    closedir($handle);
  }