PHP reuses connections, if possible. When a connection is being made to the same Host with the same Username and Password as an existing connection, no second connection is being made by PHP. Instead the existing connection is returned to the caller. This is true for both the *_connect() and *_pconnect() calls of all PHP database interfaces.

This has implications for MySQL users: Never use the MySQL "use" command to change the current database. If you do, session management will fail to operate properly. Instead, create all PHPLIB tables as part of your application.

Some databases (for example Oracle) have very expensive connect() operations. For these databases, performance is dramatically improved if you switch from CGI PHP to mod_php. This is because, for these database, PHPLIB uses the persistent connection methods (e.g. *plogon() or *_pconnect()) to connect to your database. In mod_php, the database connection is kept around by the web server process after the page has been processed and is reused if a further connect requires a connection with the same Host/Username/Password pattern.

This means that there will be at most "number of web server processes" times "number of Host/Username/Password-combinations" many simultaneous connections to your database server. Keep that in mind when planning licenses and server load. Using CGI PHP and/or non-persistent connections will probably reduce the number of concurrent connects to your database server at the expense of connection setup time. For database servers where connection setup time is negligible (MySQL for example) this is the default solution; just don't try it with Oracle.

Note: PHPLib now uses (as of 7.4) non-persistent connections by default in the non-Oracle db interfaces. The majority of PHPLib users are operating in a virtual hosting environment where persistent connections eat up memory at a horrifying rate. For web servers that only connect to one Host/Username/Password, the persistent connections can be enabled by setting the $PConnect variable in local.inc's database extension class.

Note: When using persistent connections with MySQL, the wait_timeout value should be lowered in the MySQL server (safe_mysqld). A recommended value of 120 and 180 seconds is much better suited to use with PHP than MySQL's default value of 28800 seconds (8 hours).