Howto add custom php code to a template?
 

^^ ..or make a new php variable so that i can use it in my template?

Depending on the template, you will have to create a plugin hooking at the relevant location.

thanks.. which hook would be best to use for some custom php code in the header template? (global_start hook works fine, but I'm not sure if it's the best way)

ps: I read in the docs, that I should use the debug mode to find the best hook location. But I'm working in a live environment, so it wouldn't be wise to do that i guess. Is there a way to have two forums, one for the live forum and the other for debugging? or is that too complicated?

It's definitely a good idea to do your coding and debugging on a test board before it goes live. Just install a second board on your domain for testing purposes, or on your PC. I don't believe that goes against the terms of service.

For the header, global_start would be fine as both are loaded on almost every page.

thanks. :)