I decide to spend more time in refactoring. I plan to read two books thoroughly: one is "Refactoring: improving the design of existing code", and the other one is "Refactoring to patterns".
I believe refactoring can be applied to PHP applications more than others. Due to PHP language's drastic evolution from a simple script which can be embedded in HTML, to a full OOP features contained programming language, we can apply refactoring technique to tons of legacy PHP applications.
A pity is there are few refactoring topics regarding to PHP. One of them is a chapter in book "PHP In Action". However, based on my opinion, that chapter only touches the super surface of refactoring. There are serveral articles or tutorials as well, i think. But none of them dig more in this area.