Array inside array (comment system with 2 levels)
Depends on how your data is modeled. I dont know that, but I can assume that you use three relational tables. Something along these lines:

Post [parent]
+-------------+---------+
|    Field    |  Type   |
+-------------+---------+
| id          | int     |
| value       | varchar |
+-------------+---------+

Comment [parent-child]
+-------------+---------+
|    Field    |  Type   |
+-------------+---------+
| id          | int     |
| foreign_key | int     |
| value       | varchar |
+-------------+---------+

Comment-&gt;Comment [child]
+-------------+---------+
|    Field    |  Type   |
+-------------+---------+
| id          | int     |
| foreign_key | int     |
| value       | varchar |
+-------------+---------+


You are looking to potentially run three queries. An easy fix would be to add a has_children field to each parent table. With that you would save a potential checkup and easily release some complexity in the PHP loop. 

    [parent-child]
    +--------------+---------+
    |    Field     |  Type   |
    +--------------+---------+
    | id           | int     |
    | foreign_key  | int     |
    | has_children | bool    | &lt;===
    | value        | varchar |
    +--------------+---------+


I can provide the PHP code if you like, but I'm assuming that you get the general idea.