mysql - Available for PHP versions 4 and 5, this extension is intended for use with MySQL versions prior to MySQL 4.1. This extension does not support the improved authentication protocol used in MySQL 4.1.1 and later, nor does it support prepared statements or multiple statements. If you wish to use this extension with MySQL 4.1 and more recent versions of MySQL, you will likely want to configure the MySQL server to use the --old-passwords option (see Sección A.2.3, “Client does not support authentication protocol”). This extension is documented on the PHP Website at http://php.net/mysql.

mysqli - Stands for "MySQL, Improved", this extension is available only in PHP 5. It is intended for use with MySQL 4.1.1 and later. This extension fully supports the authentication protocol used in MySQL 4.1.1 and subsequent MySQL releases as well as the Prepared Statements and Multiple Statements APIs. In addition, this extension provides an advanced, object-oriented programming interface. You can read the documentation for the mysqli extension at http://php.net/mysqli. A helpful article can be found at http://www.zend.com/php5/articles/php5-mysqli.php.