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If you’re connecting to a MySQL database in your PHP
application, you’ll find that there are two alternative
connctions – mysql_connect which establishes a new
connection “each time” and mysql_pconnect which uses
persistant connections. Which should you use?
Mysql_connect opens a new connection each time a PHP page is
called up, and closes the connection down again at the end
of the request. It’s ideal for pages that don’t have a heavy
usage – doesn’t need tuning, is straightforward internally.
If you compare MySQL to a shop, this is the connection that
you would use for a small shop where the door is opened each
time a new customer wishes to enter.
Mysql_pconnect will also open a new connection when a PHP
page is called up (at any rate, it will the first time after
a server reboot), but it will NOT close the connection at
the end of the request – instead, it will save it in a
connection pool so that a subsequent request can continue to
use the same connection. It’s intended for pages that do
have a heavy usage – where the resources burn up by opening
and closing connections every time might have a severe
effect on performance. If your local supermarket had a door
that was opened each time someone went in and out, there
would be a lot of needless opening and closing going on –
better to leave it open and let a whole lot of people in and
out at the same time.
But mysql_pconnect does require some tuning of the servers
and you may need to limit the numbers of connections /
children and configure timeouts and how to deal with idle
children. The PHP website includes an overview of this to
get you started.
Finally, it is worth stressing what a persistant connect
does NOT give you. It does NOT give you sessions. It does
NOT give you a per-site-visitor login. It does NOT give you
any extra functioanllity. What it does give you – in the
right circumstances – is an efficiency improvement.