This is a difficult topic for book recommendations, but there are two that spring to mind: Advanced PHP Programming by George Schlossnagle (already recommended, I know, but it is very good), and Essential PHP Tools: Modules, Extensions, and Accelerators by David Sklar. Both of these guys are real mines of knowledge and great fun to talk to: if you see them at a conference, do try to say hello because you'll learn a lot just by listening.
If you find yourself now wanting much more information about how the internals of PHP works, then you've little choice out there except Advanced PHP Programming: George goes into a massive amount of depth about how PHP and the Zend Engine work internally, and his book is currently the definitive resource for developers who want a firm grip in the insides of PHP.
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