This book applies both to PHP 5 and PHP 7 (no PHP 6 exists) fairly well because of the large amount of overlap between the versions. This book is meant to build up experts at PHP development. It handles everything ranging from cookies, to APIs, to extending PHP via C. At 650-pages, it's a heavy read. There are lots of code examples as well as implementations of features (e.g., user authentication).

There is little that is cutting-edge in this book. It covers technology that still is useful but has been out for a while (hence the 2004 copyright). It falls short of being a classic. Nonetheless, the skills contained in this book are still useful to contemporary PHP development that continues to this day.

In particular, I found the benchmarking section helpful to my own PHP development. Schlossnagle taught me how I can track how much CPU time particular pages are using in my applications. This may not be useful for low-level websites, but it is helpful for larger applications that do some computation or database calls as a part of their feature set.

This book also has helpful texts suggested for further reading. Mainly books, not papers, are cited in these references. As a compulsive bibliophile, I like picking up on references to read further down the road.
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