<p>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.</p>

<p>PS - any recommendations regarding recommended source control/IDEs would also be great, but I guess that's a separate question.</p>

For PHP books you can check
this
question, personally I like
Programming PHP
, IDEs, I can recommend you to try:
Eclipse PDT
Zend Studio for Eclipse
NetBeans for PHP
DevPHP
Vim
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.