If i understand you well enough it seems like your talking about template files.
PHPBB im pretty sure use them as a .tpl extention
Basically what they do is they have ordinary html in them
And then you may find something like this
{tpl} or {$tpl}
That is what gets changed when PHP Processes it, and finally outputs it all..
There are also another way of doing the same thing.
VBulletin use the database to store template files. In fact everything you see on this page is in the Database right now.
It still uses the {tpl} and even $tpl in the code. But it doesnt have the need of actual template files.
The good thing about doing it the VB way is you can create multiple styles.
But the bad thing is the more templates and things Forum Information the bigger the database.
So you may wanna look at how others have attempted the Template scheme if you are wanting to make a site that way.