The mysqlnd user handler plugin can be understood as a client-side proxy for all PHP MySQL extensions (mysqli, mysql, PDO_MYSQL), if they are compiled to use the mysqlnd library. The extensions use the mysqlnd library internally, at the C level, to communicate with the MySQL server. PECL/mysqlnd_uh allows it to hook many mysqlnd calls. Therefore, most activities of the PHP MySQL extensions can be monitored.
Because monitoring happens at the level of the library, at a layer below the application, it is possible to monitor applications without changing them.
On the C level, the mysqlnd library is structured in modules or classes. The extension hooks almost all methods of the mysqlnd internal connection class and exposes them through the user space class MysqlndUhConnection. Some few methods of the mysqlnd internal statement class are made available to the PHP user with the class MysqlndUhPreparedStatement. By subclassing the classes MysqlndUhConnection and MysqlndUhPreparedStatement users get access to mysqlnd internal function calls.
The internal mysqlnd function calls are not designed to be exposed to the PHP user. Manipulating their activities may cause PHP to crash or leak memory. Often, this is not considered a bug. Please, keep in mind that you are accessing C library functions through PHP which are expected to take certain actions, which you may not be able to emulate in user space. Therefore, it is strongly recommended to always call the parent method implementation when subclassing MysqlndUhConnection or MysqlndUhPreparedStatement. To prevent the worst case, the extension performs some sanity checks. Please, see also the Mysqlnd_uh Configure Options.