PHP/MySQL question - custom forum

[Hakaslak]

OK. First off. I bought vb. I use vb. Please do not flame me. I'm tired of getting stupid responses like "Just use [insert forum package i.e vB]."

Sorry heh hehe.

I am trying to put together my own forum.

I have a MySQL table called "posts". it has a field called "post_thread_id". Now, if I didn't want to create a separate threads table, how could I set it so that when a new thread is created, "post_thread_id" is set to auto_increment, but when it is a reply to an existing thread, it would use $_GET to grab the current thread_id? I don't think PHP can modify MySQL table attributes on the fly, and I don't know how to use PHP to generate a consistent stream of auto_increment numbers.

Something like:

PHP Code:

if (isset($_GET['thread_id']))
{
    $thread_id = $_GET['thread_id'];
}
else
{
    $thread_id = auto_increment;
} 


I am coding this all assuming that the url of a post would be similar to vb and phpBBs.

I'm doing this for the learning experience. I mean, I wanted to see how hard it would be to code a forum.