I've got the bare minimum to handle the Linux/MySQL parts of the stack, but I want to have a basic grasp of PHP before I dive in. I'll be working on WordPress plugins, in case there's anything specifically interesting for that.
PS - any recommendations regarding recommended source control/IDEs would also be great, but I guess that's a separate question.
For PHP books you can check this question, personally I like Programming PHP, IDEs, I can recommend you to try:
Vim actually it's not an IDE, but you can get omnicompletion, syntax coloring, debugging, and so on...
The PHP Manual it's also a really good place to start learning the language.
The best php/mysql book is probably PHP and MySQL web development but if you already have an experience in web development you might just need something like the PHP cookbook.
Also, if you don't have any experience with share hosting, you should read something like Apache security (chapter 6).
When you're developing C# you should definately take a look at VS.Php