So how much php do you need to know to code a forum theme? By how much php, I mean how long do you need to learn php given you have no previous knowledge with it.
For a theme, you'd need to know much more CSS and HTML. In fact, I think you'd barely need PHP unless you were using software that didn't have built-in theme support. To make a decent theme though, you'd need to be pretty proficient with CSS (CSS3 would probably help), HTML would be a requirement for modifying templates and probably a decent knowledge of JavaScript (and perhaps jQuery if you were planning on a more responsive design with animations).
Well depends. If you want to start from 0 you need a lot more than just coding. If you just want to edit a theme you could do it whit basic knowledge 😀
At least on a forum, PHP is more helpful for "building" content, not displaying it. Most themes are taking the same content and essentially moving it, and styling it. Occasionally, it would help to know PHP if you wanted to do something out of the ordinary, but that would be a pretty rare occasion.
While there are a lot of designers who know PHP and HTML/CSS, there are actually quite a few that only know one or the other. And even beyond that, there are some designers who will give you your theme in a PSD, but you need to get it coded by someone else. Whether they don't actually know how to code or not is another thing, but I'd probably guess it's not that fun of a task. 😛