The chapter I wrote on PHP in O'Reilly's Webmaster in a Nutshell covered up to version 3.0.8. PHP 4 includes a complete rewrite of the underlying scripting engine based on the Zend engine from Zend Technologies . It also includes major enhancements to most of the extensions. PHP 4 will be completely threadsafe and will include support for embedding PHP into more Web servers. A full list of changes will be available when PHP 4 is released later this year.
Beta versions are currently available.
I'm also working on a book that will cover PHP 4. A recent E-Soft survey indicates that PHP is now the most popular Apache module. I think it is mostly because the barrier of entry to PHP development is low. It is also free and bundled with all the popular free Unix distributions, so a lot of people have access to it. People tend to try what they have at hand before looking for alternatives.