A few weeks ago I received David Sklar's book 'Essential PHP Tools' with the request to review it as it has some chapters on programs that I wrote.
Chapter 11 deals with understanding PHP internals and features VLD . With VLD examples showing op-codes are made to provide a better insight into how PHP works.
Chapter 12 totally deals with (version 1.3.0) of Xdebug . First the tracing and profiling features are explained and at last the remote debugging features. Because most of there features are changed in the current development version, a 2nd edition will have to be updated drastically.