Connection pool

The plugins connection pool is created when PHP
initializes its modules (MINIT) and free’d when PHP shuts
down the modules (MSHUTDOWN). This is the same as for
persistent MySQL connections.

Depending on the deployment model, the pool is used
for the duration of one or multiple web requests. Network
connections are bound to the lifespan of an operating system level
process. If the PHP process serves multiple web requests as it is
the case for Fast-CGI or threaded web server deployments, then the
pooled connections can be reused over multiple connections. Because
multiplexing means sharing connections, it can even happen with a
threaded deployment that two threads or two distinct web requests
are linked to one pooled network connections.

A pooled connection is explicitly closed once the
last reference to it is released. An implicit close happens when
PHP shuts down its modules.