Sometimes, PHP "as is" simply isn't enough. Although these cases are rare for the average user, professional applications will soon lead PHP to the edge of its capabilities, in terms of either speed or functionality. New functionality cannot always be implemented natively due to language restrictions and inconveniences that arise when having to carry around a huge library of default code appended to every single script, so another method needs to be found for overcoming these eventual lacks in PHP.
As soon as this point is reached, it's time to touch the heart of PHP and take a look at its core, the C code that makes PHP go.
This information is currently rather outdated, parts of it only cover early stages of the ZendEngine 1.0 API as it was used in early versions of PHP 4.
More recent information may be found in the various README files that come with the PHP source and the Internals section on the Zend website.