PHP Includes and external php files

[php4ever]

I read threads that say hacking vBulletin will alter support privileges and yet I see others that say using php includes is not proper either considering the vBulletin template system. What I want to know is this;

Where are there any guides, tutorials or requirements on creating products and plugins?

Which is the better method to use? products and plugins or phpincludes?

I've got code that switches content based on browser written in PHP and should work fine as an include but standard phpinclude methods dont work.

Any direction would be appreciated.