The only header file you really have to include for your
modules is php.h, located in the
PHP directory. This file makes all macros and API definitions
required to build new modules available to your code.
Tip: It's good practice to create a separate
header file for your module that contains module-specific
definitions. This header file should contain all the forward
definitions for exported functions and include php.h. If you created your module using ext_skel you already have such a header
file prepared.