Header File Inclusions
    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.