I was asked to do some development with php5 (extending existing modules).
I have experiences with Java EE, JSF2, general webapps. But did not code any php, not even a bit.
How difficult is it to learn php5? What are the main differences which I should be aware off?
I am so sorry to hear this. If this happen to me, I can imagine!!! Seems like it happened to you because of the “Lack of Developers” in your working place.
Anyway, PHP is not hard as Java, because it is not a programming language but a highly advance scripting language. However, you might feel it as boring too.
You can go to w3schools, refer php manual and of course PHP books like “Head First PHP and My SQL”. But your question made me to think they are asking for a high level development. If yes, then you need time, or you have to talk with them, because you know, nothing can be learn by one day, because we are humans.
I strongly recommend studying a book, because online resources will teach you here and there, and most probably, the basics only. Once you learn it from a GOOD material, then of course you can find advance php tutorials (specially answers to questions).