Like all good books, you should read this even if you never plan to follow the title. What you will gain from the text is a better understanding of PHP and knowledge about how a large program (PHP itself) has been designed and works. I'm reminded of Lions' Commentary on Unix (though Sara's book does not reproduce all the code of PHP!) because this is a ground breaking book.
It's always good to get a better idea of what's going on behind the scenes. Too many people just use PHP and think new versions just magically appear. In truth, there are a huge number of developers working on new features and fixing bugs to help make PHP, the language, a better place. And, thanks to Sara, the work those folks do gets a bit more recognition.