This is such a good book that I wanted to take the time to write a review and give it 5 stars, which it richly deserves. The other glowing reviews helped me to choose this book over others, and they were right on the money.
I'm an experienced C/C++ programmer on Windows, but I knew nothing about PHP and MySQL -- and very little about Apache and Unix -- prior to opening this book. I've now read about 80% of it and, in the process, I've built a commercial-grade Website with user registration and shopping cart facilities, which was my objective. Reading this book was a very efficient use of my time -- it gave me exactly what I needed to build a practical Web application system with PHP and MySQL, and very little extraneous stuff.
The main prerequisite for this book is a working knowledge of HTML, and just a little background in procedural programming. Some of the earliest examples use HTML tags for tables and forms, with PHP use thoroughly explained, but without many notes on the HTML. A beginning programmer can learn effectively from this book, but as an experienced programmer I felt that it also worked well to bring me up to speed quickly on a new language.
Another value of this book not mentioned in other reviews are the many good recommendations for organizing your PHP code (applying basic software engineering principles) as your Web application gets larger and more complex. Many, many Websites have been built haphazardly and are now difficult to maintain because they haven't followed the excellent advice in this book.
I did notice the typos mentioned by other reviewers, but after reading 80% of this (867-page) book I feel they are very minor and really do not detract from the book significantly at all.
All in all, this is one of those rare books that is probably worth ten times the amount that you pay for it, and much more if you use it effectively.