mtree.php

function whitelist($parent, $whitelist = null) {
  global $drive, $shared_folder;

  if (!$whitelist) {
    $whitelist = array($parent);
  }

  $search = $drive->search("'$parent' in parents");

  foreach ($search->files as $file) {
    if ($file->type == 'folder') {
      $whitelist[] = $file->id;
      $whitelist = whitelist($file->id, $whitelist);
    }
  }
  return $whitelist;
}
$whitelist = whitelist($shared_folder);