It won't be the first book you buy for learning PHP, but it could be the second.
Core PHP is quite a large book, with over a thousand pages. It is part of Prentice Hall's "Core..." series, which generally have a high level of quality, and this book is no exception. The production quality is good, the writing style is clear and there are very few typo's. Leon Atkinson has the knack of writing in short clear sentences, yet still maintaining readability.
The book can be considered in three main sections. The first section of about 160 pages covers the basics of PHP, the core language and structures and the rudiments of network connectivity. The second section is the main reference work, of about 770 pages and the final section, of about 90 pages covers general software engineering with PHP.
The meat of the book is really the middle section, which taken on its own would be a good reference work. The first and third sections cover such different audiences that it distracts from the focus of the book. Is it a book for beginners, a reference work or "PHP Best Practices"? Given its size it may have been better to have made two books out of it, one as a reference, one as a programmers guide. The size of the book makes it unwieldy for quickly looking up information.
My main complaint though is that the reference section does not differentiate between functions that have been introduced in PHP 5 and functionality that existed before then. This is important in a reference work, especially as PHP 5 will not be in general use for another year or so. The online PHP references all indicate which version of PHP first used a particular function and it would have been easy to do the same here. I can only guess that the intent is to spur people on to using PHP 5, but it does detract from the usefulness of the reference.
PHP 5 is certainly a major step forward for PHP, which is already a robust and well-established language on the internet. Unfortunately, as it is very well established, most ISPs and hosting companies are going to hesitate before doing a major upgrade to PHP 5, which is why I feel that it may be a year or so until it is in common use.
Reading the book certainly gives an exciting taste of the future of PHP and, although the book could have been better, it still gets a rating of Recommended. It won't be the first book you buy for learning PHP, but it could be the second.