I came across this interesting PHP reading list compiled by IBM. There is too much online material about PHP to read, you may get lost trying to figure out what is worth reading and what is not, where to go for specific information you want or from where to start. This guide provides you with a road map of some of the materials about PHP that are worth reading. It also organizes things so that you can know where to go when you are looking for some specific thing.
IBM is a great supporter for open source. Google is another strong supporter of open source as well. I wonder when or if ever will Microsoft follow suit. Probably not.