This book assumes no PHP programming skill at all - you'll be taught from scratch in that respect. However, I will be using terms like "variable", "function", and "loop" freely, so any prior programming experience you have will help enormously.
If you're coming from the world of HTML, PHP is a great step forward to make. HTML by itself, as you may have discovered, is a very simple language, which as a result leaves you with very simple pages. Think of using HTML like owning a car with no engine - the car might look great, but it will never actually do very much. PHP can take your sites into the fast lane by allowing you to turn your static pages into exciting, ever-changing pages with only a little thinking. In essence, it's like putting a V6 engine into your car shell.
Programmers coming from the world of Perl, you should prepare yourself for a surprise: Perl isn't the only good language out there! PHP is much easier on the eye than Perl, and often faster both in terms of development time and execution time. However, you have to make one big sacrifice: the major difference between PHP and Perl is that PHP has much less "one-line magic" than Perl. Regular contributors to the Obfuscated Perl Contest may find that PHP is probably not for them!
Those of you already programming in PHP and looking to extend your knowledge, then this book is ideal - you will find information on how to use the more advanced features (IMAP, XML, and Sockets), tips and tricks on how to program better PHP, and a great reference too.
If you're not a HTML programmer and not a Perl programmer, then relax - all the HTML and CGI information you need to know to use PHP is covered here.
Please note: this book has been written to be OS-agnostic. That is, samples are included for both Windows and Unix-like systems where possible. However, due to operating system constraints some parts that work on Windows may not work on Unix OSs, and vice versa. These sections are marked individually, along with reasoning as to why the information is not cross-platform.
Finally, if you're expecting that it is easy to learn PHP; that you can learn a new language without having to memorise hundreds of functions; that one book can take you from beginner to expert in web development - you're absolutely right, because you're reading it!