Best table structure for multi-section, nested comments

I'm hoping you experts can help me think through the best approach for something I am working on.

On the site, we have 3 sections that contain various details about specific phases (Phase 1, Phase 2, Phase 3) of a project. Users will have the ability to post comments in each of the sections. They will also be able to make comments on previous comments, but only 1 nested level deep.

So:
===================
Project 1

Phase 1
Phase 2

Comment 1

Comment 1a

Comment 2

Phase 3

Project 2

Phase 1

Comment 1

Comment 1a
Comment 1b

Comment 2

Comment 2a

Phase 2
Phase 3

===================
Etc.

To handle this, I am thinking that my table will need to contain the following columns:

===============
| comments.tbl |
===============
| comment_ID |
| project_ID |
| phase_ID |
| top_ID |
| user_ID |
| comment_ts |
===============

WHERE
comment_ID is the primary key
project_ID ties to the project ID stored in the projects.tbl
phase_ID indicates which phase section this comment belongs
top_ID if not null references the comment_ID in this table that this comment is in reference to
user_ID ties to the user ID stored in the users.tbl for the poster
comment_ts stores the timestamp when the comment was posted

Assuming that is the correct structure (which I'm sure you'll tell me if it's not), how would I best query this table in PHP to output my comments in the format shown above. I know I would put the project_ID and phase_ID in the WHERE clause to narrow it down that far. But how would I show the comments nested appropriately:

===============
Comment 1

Comment 1a
Comment 1b

Comment 2

Comment 2a

===============

I'm not a fan of "what's the best..." type questions - I think they're kind of sneaky! That said, it's probably kindest if you don't now search these forums for examples of where I've done exactly that!

I think this structure's OK. The only problem with this model is that there's a redundancy: a child comment MUST belong to the same project and same phase as its parent. In these instances, it's probably best to separate the 'response comments' out into a separate table.

Also, your table may want to include a column for the actual comment!

Good call, of course you are right here.



Hee hee... Whoops!

So once I break this into two tables, can you (or someone) help me with the query structure to loop through the results and output the nested comments? If you do a search for my name, you'll see I do a really good job at screwing up JOINs, and I'm trying to save myself a little bit of preemptively.

EDIT: Actually, maybe I don't need the JOIN at all. Maybe it's two separate queries, with an IF statement within the first one, checking to see if there are any associated nested comments. Hmmm... will have to try that.