Personally I would recommend learning as many different things as you can. I recommend reading a really interesting article by Carlos Bueno, who works for Facebook, about the Full Stack Programmer.
It's a pretty big read, but the main idea is that to write really good code, you know to understand as much about the system as possible. That doesnt mean just learning as much as you can about PHP, but also Linux, Apache, MySQL and anything else that you use in your site. While you may learn a lot about PHP (or any language you choose to work in), you also need to understand why you write code the way you do, or how to analyse the code to make it better. Otherwise, you will have issues with performance.
From my experience, there are those who stick with one technology, refuse to look at others, and generally their projects suffer for it. Learn as many different things as you can.