I've tried using nested sets, and they become very difficult to maintain when dealing with multiple trees and lots of other complications.. I'd like to give PHP's SPL library a stab at this (btw, we are PHP 5.3, MySQL 5.1).

Given two datasets:

The Groups:

+-------+--------+---------------------+---------------+
| id    | parent | Category Name       | child_key     |
+-------+--------+---------------------+---------------+
| 11133 |   7707 | Really Cool Products| 47054         |
|  7709 |   7708 | 24" Monitors        | 57910         |
|  7713 |   7710 | Hot Tubs            | 35585         |
|  7716 |   7710 | Hot Dogs            | 00395         |
| 11133 |   7707 | Really Cool Products| 66647         |
|  7715 |   7710 | Suction Cups        | 08396         |
+-------+--------+---------------------+---------------+

The Items

+------------+------------+-----------+----------+---------+
| child_key  | totalprice | totalcost | totalqty | onorder | (jan, feb, mar..)
+------------+------------+-----------+----------+---------+
| 24171      |      10.50 |     20.10 |      200 |     100 |
| 35685      |      10.50 |     20.10 |      200 |     100 |
| 76505      |      10.50 |     20.10 |      200 |     100 |
| 04365      |      10.50 |     20.10 |      200 |     100 |
| 01975      |      10.50 |     20.10 |      200 |     100 |
| 12150      |      10.50 |     20.10 |      200 |     100 |
| 40060      |      10.50 |     20.10 |      200 |     100 |
| 08396      |      10.50 |     20.10 |      200 |     100 |
+------------+------------+-----------+----------+---------+

The figures are actually much more complicated than this (I am actually aggregating a variable amount of months or years over the past 15yrs, so there may need to be 20 columns of aggregated results).

I have been trying to figure out RecursiveIterator and IteratorAggregate, but I am having a difficult time finding real world examples that are generic enough to really wrap my head around these classes.

Can someone give me a head start?