They’re designs which do not contain code for a specific CMS .
CMS themes have their own category (or go in Misc).
yup, yup. What danharper and BroOf said!
You can also use PHP for other elements like server side includes, PHP navigation, PHP based photo galleries, etc. BUT be aware that many people aren’t familiar with PHP and that may deter them from buying your template. It’s a good idea to include two versions of the template. One without PHP (except for maybe the contact form) and one with the PHP (like ssi’s, nav, etc.).
It doesn’t take much extra work and it gives potential buyers more options. They love the options!
BTW , welcome to ThemeForest!
To add on what xmdsys said, I am toying with the idea of coding out a design for just a HTML theme. I was planning on coding it in PHP and ASP so that the buyer has a choice depending on their host. I would take a pretty extensive read me file but the advantages that yohave when using a server side language far outweighs the work involved. Plus, looking at some of the themes here, even if the design isn’t all that appealing (to me), the more options you give to the buyer, it seems the more it can be sold for.
The layout has a few dynamic areas in my mind so I plan on coding those areas in PHP and ASP versons as well as a jQuery version.