Persistent connection support was introduced in PHP 5.3 for the mysqli extension. Support was already present in PDO MYSQL and ext/mysql. 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
Reset session variables
Close prepared statements (always happens with PHP)
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.