Programming PHP

Chapter 14 CHAPTER 14
Extending PHP
This chapter shows you how to write C language extensions to PHP. Although most
functionality can be written in the PHP language, sometimes you need the extra
speed and control you get from the C API. C code runs an order of magnitude faster
than most interpreted script code, and it is also the mechanism for creating the thin
middle layer between PHP and any third-party C library.
For example, to be able to talk to the MySQL database server, PHP needs to implement
the MySQL socket protocol. It would be a lot of work to figure out this protocol
and talk to MySQL directly using fsockopen( ) and fputs( ) from a PHP script.
Instead, the same goal can be accomplished with a thin layer of functions written in
C that translate MySQL’s C API, implemented in the libmysqlclient.so library
included in MySQL, into PHP language-level function calls. This thin layer of functions
is known as a PHP extension. PHP extensions do not always have to be a layer
between PHP and some third-party library, however. An extension can instead completely
implement some feature directly (for example, the FTP extension).
Before we get into the details of writing extensions, a note of caution. If you are just
learning PHP and do not have any sort of C programming background, you should
probably skip this chapter. Extension writing is an advanced topic, and it is not for
the faint of heart.
Architectural Overview
There are two kinds of extensions that you can write: PHP extensions and Zend
extensions. We will focus on PHP extensions here. Zend extensions are lower-level
extensions that somehow modify the very core of the language. Opcode cache systems
such as APC, Bware afterBurner, and ZendCache are Zend extensions. PHP
extensions simply provide functions or objects to PHP scripts. MySQL, Oracle,
LDAP, SNMP, EXIF, GD, and ming are all examples of PHP extensions.
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