I have just bought the 4th edition after using the 1st edition for years. I referred to the old book at least a couple of times a week and the new one is even better; bringing it up to date with PHP 5.3 and MySQL 5.1 but also adding more detail about things like security and AJAX. The new book is 100 pages bigger than the 1st edition.
For me this book is perfect, I'm an experienced programmer but not a PHP guru and I find the level about right. I mostly look up PHP functions etc online but if I need an explanation about where and why I might use this function I go to the book.
If you are not a total beginner and want one good PHP book I would recommend this one.