Dunno really, hard to say. I like where I am at the moment but I don't really know any massive companies that are big on PHP. I used to want to be a mechanic though and if I changed what I do and did that, I'd want to work at BMW.
Look at existing code, see what it does and how it works. Add stuff, change stuff, break stuff. Write little scripts that do one thing just for the sake if using a certain piece of code or new technique. Learn by doing.