I do a lot of PHP web development. The books that use the most are "MySQL by Paul DuBois", and this one.
I hear a lot of comments stating, "you can find it online". Yes, its true, you can. But, when you are writing code the last thing you want is another window open or spend valuable time trying to find an answer.
The authors have done a great job laying this book out. The information is catagorized and aphabetized. There are short examples that do a good job of explaining how each function works. And what really sets it apart from others, is it tells you if the function has been deprecated and/or what version of PHP and OS supports it, it has a "see also" for each pertinent function, and a great layout(check out the sample pages).
If you program using PHP, get this book. The time it saves is well worth the price. This book is for all levels of PHP programmers, novice to experts. Don't be fooled into thinking all you need is the downloadable manual from php.net. The manual is useful, but it doesn't work as well as this book.
This book is always within arms reach when I'm programming and I'm sure it will be the same for you.