Those who know don't talk.
Those who talk don't know.
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This information is currently rather outdated,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
section on the Zend website.