[Lynne]

In vbulletin, you would use a plugin to 'plug into' the php file. You plug into a hook. If you look at the forumdisplay.php file, you will see several lines like this:
($hook = vBulletinHook::fetch_hook('forumdisplay_start')) ? eval($hook) : false;

That is a for a plugin. So, if you write a plugin using the hook location 'forumdisplay_start' then the php code is 'entered' right there in the file.

Then, after you write the plugin, you display the results in a template file. For forumdisplay.php, the main template file is FORUMDISPLAY.