$tree = array();
foreach ($list as $id => $row) {
 $pid = $row->pid;
 if ($pid == 0) {
  $tree[] = &$list[$id];
 } else if (isset($list[$pid])) {
  $parent = &$list[$pid];
  $parent->children[] = &$list[$id];
  // 增加 children 属性标识子分类列表
 }
}