# The mysqli Extension and Persistent Connections

The idea behind persistent connections is that a connection between a client process and a database can be reused by a client process, rather than being created and destroyed multiple times. This reduces the overhead of creating fresh connections every time one is required, as unused connections are cached and ready to be reused.

Unlike the mysql extension, mysqli does not provide a separate function for opening persistent connections. To open a persistent connection you must prepend `p:` to the hostname when connecting.

The problem with persistent connections is that they can be left in unpredictable states by clients. For example, a table lock might be activated before a client terminates unexpectedly. A new client process reusing this persistent connection will get the connection as is. Any cleanup would need to be done by the new client process before it could make good use of the persistent connection, increasing the burden on the programmer.

The persistent connection of the `mysqli` extension however provides built-in cleanup handling code. The cleanup carried out by `mysqli` includes:

- Rollback active transactions

- Close and drop temporary tables

- Unlock tables

- Reset session variables

- Close prepared statements (always happens with PHP)

- Close handler

- Release locks acquired with `GET_LOCK`

This ensures that persistent connections are in a clean state on return from the connection pool, before the client process uses them.

The `mysqli` extension does this cleanup by automatically calling the C-API function `mysql_change_user()`.

The automatic cleanup feature has advantages and disadvantages though. The advantage is that the programmer no longer needs to worry about adding cleanup code, as it is called automatically. However, the disadvantage is that the code could potentially be a little slower, as the code to perform the cleanup needs to run each time a connection is returned from the connection pool.

It is possible to switch off the automatic cleanup code, by compiling PHP with `MYSQLI_NO_CHANGE_USER_ON_PCONNECT` defined.

The `mysqli` extension supports persistent connections when using either MySQL Native Driver or MySQL Client Library.