The PHP MySQL extensions are lightweight wrappers on top of a C client library. There are 3 PHP MySQL extensions:
It is recommended to use either the mysqli or PDO_MySQL extensions. It is not recommended to use the old mysql extension for new development.
The extensions can either use the mysqlnd or libmysql library to connect from PHP to MySQL. Choosing one or the other library is a compile time decision. Both libraries are supported and constantly being improved.
MySQL recommends using the MySQL native driver for PHP (mysqlnd) together with ext/mysqli or PDO_MySQL.
All 3 PHP MySQL extensions and the mysqlnd library are part of the source code of PHP. The source code and binaries are available from: http://php.net/downloads.php
As of PHP 5.4, the mysqlnd library is a php.net compile time default to all PHP MySQL extensions. Also, the php.net Windows team is using mysqlnd for the official PHP Windows distribution since mysqlnd became available in PHP 5.3.
The mysqlnd library is highly optimized for and tightly integrated into PHP. The MySQL Client Library cannot offer the same optimizations because it is a general-purpose client library.
The mysqlnd library is using PHP internal C infrastructure for seamless integration into PHP. In addition, it is using PHP memory management, PHP Streams (I/O abstraction) and PHP string handling routines. The use of PHP memory management by mysqlnd allows, for example, memory savings by using read-only variables (copy on write) and makes mysqlnd apply to PHP memory limits. Additional advantages include:
A wide range of mysqlnd plugins, providing additional features are available from PECL. Popular mysqlnd plugins include: