How would it be best to create forum? Using minimal php and mostly html? Or php?
I think you have mis-understood the meaning and objectives of HTML and PHP.
HTML is only for presentation and layout the structure of contents.
PHP is the programming language to actually make those 'static' HTML into a 'dynamic' page.
Now to reply to your question, you need to use BOTH.

PS: You could try using and experimenting with opensource forums and get the feel of how things work.. It's very helpful.
Cheers!