Files which make up an extension
The zend_module structure
Extension structure
PHP Manual

## Basic constructs

C is a very low-level language by modern definitions. This means that it has no built-in support for many features that PHP takes for granted, such as reflection, dynamic module loading, bounds checking, threadsafe data management and various useful data structures including linked lists and hash tables. At the same time, C is a common denominator of language support and functionality. Given enough work, none of these concepts are impossible; the Zend Engine uses them all.

A lot of effort has gone into making the Zend API both extensible and understandable, but C forces certain necessary declarations upon any extension that to an inexperienced eye seem redundant or plain unnecessary. All of those constructs, detailed in this section, are "write once and forget" in Zend Engine 2 and 3. Here are some excerpts from the pre-generated php_counter.h and counter.c files created by PHP 5.3's ext_skel, showing the pre-generated declarations:

> Note: The astute reader will notice that there are several declarations in the real files that aren't shown here. Those declarations are specific to various Zend subsystems and are discussed elsewhere as appropriate.

    extern zend_module_entry counter_module_entry;
    #define phpext_counter_ptr &counter_module_entry

    #ifdef PHP_WIN32
    #	define PHP_COUNTER_API __declspec(dllexport)
    #elif defined(__GNUC__) && __GNUC__ >= 4
    #	define PHP_COUNTER_API __attribute__ ((visibility("default")))
    #else
    #	define PHP_COUNTER_API
    #endif

    #ifdef ZTS
    #include "TSRM.h"
    #endif

    #ifdef HAVE_CONFIG_H
    #include "config.h"
    #endif

    #include "php.h"
    #include "php_ini.h"
    #include "ext/standard/info.h"
    #include "php_counter.h"

    /* ... */

    #ifdef COMPILE_DL_COUNTER
    ZEND_GET_MODULE(counter)
    #endif

- The lines concerning counter_module_entry declare a global variable, and a macroed pointer to it, which contains the zend_module_entry for the extension. Despite the later discussion regarding the drawbacks of "true" globals, this usage is intentional; Zend takes precautions to avoid misusing this variable.
- `PHP_COUNTER_API` is declared for use by non-PHP functions the module intends to export for the use of other modules. The counter extension doesn't declare any of these, and in the final version of the header file, this macro has been removed. The `PHPAPI` macro is declared identically elsewhere and is used by the standard extension to make the phpinfo() utility functions available to other extensions.
- The include of TSRM.h is skipped if PHP, or the extension, isn't being compiled with thread-safety, since in that case TSRM isn't used.
- A standard list of includes, especially the extension's own php_counter.h, is given. config.h gives the extension access to determinations made by configure. php.h is the gateway to the entire PHP and Zend APIs. php_ini.h adds the APIs for runtime configuration (INI) entries. Not all extensions will use this. Finally, ext/standard/info.h imports the aforementioned phpinfo() utility API.
- `COMPILE_DL_COUNTER` will only be defined by configure if the counter extension is both enabled and wants to be built as a dynamically loadable module instead of being statically linked into PHP. `ZEND_GET_MODULE` defines a tiny function which Zend can use to get the extension's zend_module_entry at runtime.

  > Note: The astute reader who has peeked into main/php_config.h after trying to build with the counter module enabled statically may have noticed that there is also a `HAVE_COUNTER` constant defined that the source code doesn't check for. There's a simple reason this check isn't done: It's unnecessary. If the extension isn't enabled, the source file will never be compiled.

Files which make up an extension
The zend_module structure
Extension structure
PHP Manual