Extending and Embedding PHP
Sara Golemon

£45.86

buy from amazon

I can't claim to have read the whole of this book - but I have found parts of useful on occasions when I needed that deeper insight into how PHP works.

I have found it well written and very approachable for such a deeply technical book.

There doesn't seem to be much to compete with it - though please comment if I've missed something.

It's definitely on my TODO list for further reading.