CSS is not a programming language, it's a language site layout and design.
It's client side, PHP is server-side, i.e. the files are processed on the server before being sent to the browser rather than the browser interpreting the file.
The way I learnt PHP was to find a script that does something you want to do and look at it, pick it apart, improve it etc.
I also set myself little projects........
I currently have for my personal use scripts that list my DVD collection with search facility and I can put in when I watch a DVD etc and a similar sort of thing for my DVD collection, as well as a website photo gallery script
Most of all don't be scared.........it may look daunting but once you get the hang of it you'll find it pretty straight forward.
Oh yeah, and the best piece of advice.......if you get totally stuck, don't hesitate to post here......we don't bite. I've had loads of advice from here, I'm a relative newbie having only been PHPing since March and on a totally amateur basis.