The mysqlnd user handler plugin can be understood as a client-side proxy
for all PHP MySQL extensions (mysqli,
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
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.
mysqlndfunction 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.