php 5.3开始使用mysqlnd作为的默认mysql访问驱动
mysqlnd成为php 5.3中的默认mysql驱动，它有如下优点：
- mysqlnd更容易编译: 因为它是php源码树的一个组成部分
- mysqlnd和php内部机制结合更紧密，是优化过的mysql驱动
- mysqlnd更节省内存，从测试结果来看，比传统的mysql扩展节省40%的内存
- mysqlnd更快
- mysqlnd提供了丰富的性能统计功能
- mysqlnd使用了PHP license以避免不必要的版权纠纷
这个改动应同时对mysql和pdo_mysql扩展生效。
（ps）如果在使用php5.3.X的时候遇到这个提示：“mysqlnd cannot connect to MySQL 4.1+ using old authentication”，那么就是因为php5.3.x使用password做密码加密，而不是old_password。old_password加密后为16位的字符串，password加密的为41位的字符串。这时只需要用password重新修改下密码并使之生效就可以了。
mysqlnd是什么
mysqlnd是mysql原装的php驱动
考文章：http://forge.mysql.com/wiki/PHP_MYSQLND#A_brief_history_of_PHP.E2.80.99s_MySQL_APIs
介绍了关于mysql在PHP中的成长过程~
- PHP 3+ - ext/mysql: the oldest extension and API - please don’t use it any more, it does not support all MySQL features.
- PHP 5+ - ext/mysqli: the “current” extension and API - supports all features of MySQL
- PHP 5+ - PDO/MySQL: an extension and database API abstraction layer introduced with PHP 5
/ext/mysql /ext/mysqli /pdo/mysql都是用C写出来的扩展，也叫PHP的API，这三者都是用了libmysql（MySQL Client Library）。
mysqlnd is not a new extension! mysqlnd is it not a new API!
Mysqlnd is neither a new PHP extension nor a new API! mysqlnd is new C-level library code. The mysqlnd library provides almost the same functionality as libmysql does. Both C-libraries implement the MySQL communication protocol and can be used to connect to the MySQL Server.
However, libmysql is a generic C-library with Dual-Licensing. Any C-based program can use it. As PHP is based on C, PHP is using it. mysqlnd is not a generic C-library. mysqlnd is licensed under the PHP license and it is tightly integrated into PHP on the C-level. For example, mysqlnd is using the PHP memory management functions and network streams. Due to the close integration, it is difficult for other C programs but PHP to use the library. Any other C program that tries to use mysqlnd would need to link against large parts of PHP. Maybe this explains what “native” and “for PHP” means.
总的意思是说mysqlnd是一种新的函数库，libmysql有的功能它几乎都有，但是mysqlnd并不像libmysql那样作为通用库，它是专门为PHP而写的一个库，用了PHP的内在管理函数以及一些网络流的函数