HTML = text
CSS = styles and theming
PHP = dynamic operations, like contact forms (as mentioned by smoovo), login/logout, create/delete content, etc
HTML/CSS are pretty much standard. No matter what kind of web developer you are, you need to use HTML and CSS.
The dynamic language on the other hand, is up to you. I always look at PHP as the long time standing "classic" web programming language. It's been around for a while, and it currently its the most used language. However, I rarely code with PHP (unless I have to), because I prefer to use Ruby on Rails. I'm not saying PHP is bad (I dont want to start a flame war), I just don't prefer it. I think it's messy code and I don't enjoy coding in it. But it's still a great language for overall needs
and it's a great place to start.