I would just go pick up a couple of good PHP books at your local bookstore. Do your research first on Amazon or something to find what works for you. I suggest checking out the Sams Teach Yourself PHP4 in 24 Hours, by Matt Zandstra. It's been a great starting text to learn PHP, I found it very easy to pick up using this book. Then I suggest searching for another book if you're going to do a lot of complex tasks for an advanced reference, otherwise this book will do you fine.
I always like to suggest one learning the code behind the GUI if at all possible. I like using BBEdit as well. I wouldn't throw away your Mac to get a GUI for PHP scripting. I think the Mac has probably one of the better, if not the best, web development environments around for learning what real IT professionals do with the Unix backend and MacOS frontend. Of course, in the real world, Linux/Unix boxes with Unix backends and GUIs like RedHat, Mandrake, etc. dominate. With the new MacOS though, this transition, in either direction, is much simpler.