Where in the code are new threads created?

[Randomguyy]

So making db entries when creating a new topic works for me. Now, though, I have a new problem.

What I want to do is make it possible to basically reply to a topic with another topic. To do this, I created a button in a PHP module in the first topic, which also adds the topic's ID in the URL. I want to save the IDs of the first and second topic in the db so that I can connect both via a link in f.e. a PHP module.

Now I want this to happen right as you click the 'Post' button for creating a new topic. However, for the life of me, I can't find where exactly in the code I would have to add my own code to execute the db insert.

help pls?