... Directory and File Functions . ... Main directory layout of the PHP source tree. ... Main PHP source files and main header files; here you'll find all of PHP's API definitions, macros, etc. (important). ext . ... php.h , located in the main PHP directory. This file contains most of PHP's macro and API definitions. zend.h , located in the main Zend directory. This file contains most of Zend's macros and definitions. zend_API.h , also located in the Zend directory, which defines Zend's API. ...