Migrating from PHP 5.2.x to PHP 5.3.x
PHP Manual

New Extensions

The following are new extensions added (by default) as of PHP 5.3.0:

mysqlnd is a new core library shipped with PHP. It is a PHP-specific replacement for libmysql. mysqlnd will be used to build the mysql, mysqli and PDO_MYSQL if libmysql isnt found on the system, but it may also be used instead of libmysql.


Migrating from PHP 5.2.x to PHP 5.3.x
PHP Manual