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# Chapter 26. Source Layout

> Note: Prior to working through the rest of this chapter, you should retrieve clean, unmodified source trees of your favorite Web server. We're working with Apache (available at http://www.apache.org/) and, of course, with PHP (available at http://www.php.net/ - does it need to be said?).

> Make sure that you can compile a working PHP environment by yourself! We won't go into this issue here, however, as you should already have this most basic ability when studying this chapter.

Before we start discussing code issues, you should familiarize yourself with the source tree to be able to quickly navigate through PHP's files. This is a must-have ability to implement and debug extensions.

The following table describes the contents of the major directories.

Directory | Contents
`php4` | Main PHP source files and main header files; here you'll find all of PHP's API definitions, macros, etc. (important). Everything else is below this directory.
`php4/ext` | Repository for dynamic and built-in modules; by default, these are the "official" PHP modules that have been integrated into the main source tree. From PHP 4.0, it's possible to compile these standard extensions as dynamic loadable modules (at least, those that support it).
`php4/main` | This directory contains the main php macros and definitions. (important)
`php4/pear` | Directory for the PHP Extension and Application Repository. This directory contains core PEAR files.
`php4/sapi` | Contains the code for the different server abstraction layers.
`php4/TSRM` | Location of the "Thread Safe Resource Manager" (TSRM) for Zend and PHP.
`php4/Zend` | Location of the Zend Engine files; here you'll find all of Zend's API definitions, macros, etc. (important).

Discussing all the files included in the PHP package is beyond the scope of this chapter. However, you should take a close look at the following files:

- `php4/main/php.h`, located in the main PHP directory. This file contains most of PHP's macro and API definitions.

- `php4/Zend/zend.h`, located in the main Zend directory. This file contains most of Zend's macros and definitions.

- `php4/Zend/zend_API.h`, also located in the Zend directory, which defines Zend's API.

You should also follow some sub-inclusions from these files; for example, the ones relating to the Zend executor, the PHP initialization file support, and such. After reading these files, take the time to navigate around the package a little to see the interdependencies of all files and modules - how they relate to each other and especially how they make use of each other. This also helps you to adapt to the coding style in which PHP is authored. To extend PHP, you should quickly adapt to this style.

# Extension Conventions

Zend is built using certain conventions; to avoid breaking its standards, you should follow the rules described in the following sections.

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