"camelCase" when and where?

[eluser]IamPrototype[/eluser]
I'm just curious about the standards in different languages.

* In PHP5 all classes should use underscore and functions should be using underscore as their seperator
* In CSS all elements (such as H1), classes and ids should be written in camelcase

Maybe it doesn't count whether you're using camelcase or not, but I'm really into standards and I want to keep my code clean and friendly.

Right now, in CSS, I'm using an underscore everywhere (elements, ids, classes) and in PHP5 I'm using an underscore for my classes and functions, like this:

Code:

class Class_name
function function_name

But really.. when and where should I use camelcase? According to the standards.