The mysqlnd user handler plugin (mysqlnd_uh) allows users to set hooks for most internal calls of the MySQL native driver for PHP (mysqlnd). Mysqlnd and its plugins, including PECL/mysqlnd_uh, operate on a layer beneath the PHP MySQL extensions. A mysqlnd plugin can be considered as a proxy between the PHP MySQL extensions and the MySQL server as part of the PHP executable on the client-side. Because the plugins operates on their own layer below the PHP MySQL extensions, they can monitor and change application actions without requiring application changes. If the PHP MySQL extensions (mysqli, mysql, PDO_MYSQL) are compiled to use mysqlnd this can be used for:
Monitoring
Queries run by any of the PHP MySQL extensions
Prepared statements run by any of the PHP MySQL extensions
Auditing
Detection of databases uses
SQL Injection protection using black-/whitelists
Assorted
Load Balancing connections
As of version PHP 5.3.3 the MySQL native driver for PHP (mysqlnd) features an internal plugin C API. C plugins, such as the mysqlnd user handler plugin, can extend the functionality of mysqlnd. PECL/mysqlnd_uh makes parts of the internal plugin C API available to the PHP user for plugin development with PHP.
The MySQL native driver for PHP is a C library which ships together with PHP as of PHP 5.3.0. It serves as a drop-in replacement for the MySQL Client Library (AKA libmysql/libmysqlclient). As of PHP 5.4.0, mysqlnd is the compile time default library used by all PHP MySQL extensions.
PECL/mysqlnd_uh gives users access to MySQL user names, MySQL password used by any of the PHP MySQL extensions to connect to MySQL. It allows monitoring of all queries and prepared statements exposing the statement string to the user. Therefore, the extension should be installed with care. The PHP_INI_SYSTEM configuration setting mysqlnd_uh.enable can be used to prevent users from hooking mysqlnd calls.
Code obfuscators and similar technologies are not suitable to prevent monitoring of mysqlnd library activities if PECL/mysqlnd_uh is made available and the user can install a proxy, for example, using auto_prepend_file.
Many of the extensions functions are described very briefly. This is done intentionally to avoid content duplication. The mysqli extension is often only a thin abstraction layer on top of the MySQL C API, which the mysqlnd library provides. Therefore, the corresponding mysqli documentation respectively the MySQL reference manual can be consulted to get more information on a particular function.
The shortcut mysqlnd_uh stands for mysqlnd user handler. There may be better names. However, this was the initial choice when development started. Meanwhile, non speaking names have almost become a tradition.
Note:
The mysqlnd user handler plugin is in alpha status. Take appropriate care before using it in production environments.