This section of the manual is written for the Hacker: someone thinking about getting their hands dirty, someone who wants an understanding of internals in order to advance their PHP skills, or maybe someone looking to write the next best extension. Whatever the reason, this section will seek to provide a good understanding of the internals of PHP, how to write extensions, how to understand existing code containing mystical macros. All of the important internal functionality is documented here, enough anyway to convince you to read the source.
Out of necessity, this section assumes the reader has a working knowledge of the C programming language, and associated tools, like compilers and terminal emulators and the like. You don't need to be the next Alan Turing, but a working knowledge is none the less essential for this section to be of any use. With just that working knowledge, this section should be enough to get you well on the way to earning the title Hacker, and you do have to earn it.
The documentation in this section is current as of PHP 5.3.3, deviations in API functionality are duly noted in the appropriate sections.
This documentation is still under development. The original Zend documentation is preserved in its entirety in the Zend Engine 1 section for those who need it before this documentation is completed.