Connection pooling in PHP

I suppose you're using mod_php, right?

When a PHP file finishes executing all it's state is killed so there's no way (in PHP code) to do connection pooling. Instead you have to rely on extensions.

You can mysql_pconnect so that your connections won't get closed after the page finishes, that way they get reused in the next request.

This might be all that you need but this isn't the same as connection pooling as there's no way to specify the number of connections to maintain opened.

If an unused persistent connection for a given combination of "host, username, password, socket, port and default database can not be found" in the open connection pool, then only mysqli opens a new connection otherwise it would reuse already open available persistent connections, which is in a way similar to the concept of connection pooling. The use of persistent connections can be enabled and disabled using the PHP directive mysqli.allow_persistent. The total number of connections opened by a script can be limited with mysqli.max_links (this may be interesting to you to address max_user_connections issue hitting hosting server's limit). The maximum number of persistent connections per PHP process can be restricted with mysqli.max_persistent.

In wider programming context, it's a task of web/app server however in this context, it's being handled by mysqli directive of PHP itself in a way supporting connection re-usability. You may also implement a singleton class to get a static instance of connection to reuse just like in Java. Just want to remind that java also doesn't support connection pooling as part of its standard JDBC, they're being different module/layers on top of JDBC drivers.

Coming to PHP, the good thing is that for the common databases in the PHP echosystem it does support Persistent Database Connections which persists the connection for 500 requests (config of max_requests in php.ini) and this avoids creating a new connection in each request. So check it out in docs in detail, it solves most of your challenges. Please note that PHP is not so much sophisticated in terms of extensive multi-threading mechanism and concurrent processing together with powerful asynchronous event handling, when compared to strictly object oriented Java. So in a way it is very less effective for PHP to have such in-built mechanism like pooling.

You cannot instantiate connection pools manually.

But you can use the "built in" connection pooling with the mysql_pconnect function.

I would like to suggest PDO::ATTR_PERSISTENT

Persistent connections are links that do not close when the execution of your script ends. When a persistent connection is requested, PHP checks if there's already an identical persistent connection (that remained open from earlier) - and if it exists, it uses it. If it does not exist, it creates the link.

Connection pooling works at MySQL server side like this.

1. If persistence connection is enabled into MySQL server config then MySQL keep a connection open and in sleep state after requested client (php script) finises its work and die.
2. When a 2nd request comes with same credential data (Same User Name, Same Password, Same Connection Parameter, Same Database name, Maybe from same IP, I am not sure about the IP) Then MySQL pool the previous connection from sleep state to active state and let the client use the connection. This helps MySQL to save time for initial resource for connection and reduce the total number of connection.

So the connection pooling option is actually available at MySQL server side. At PHP code end there is no option. mysql_pconnect() is just a wrapper that inform PHP to not send connection close request signal at the end of script run.

For features such as connection pooling - you need to install swoole extension first: https://openswoole.com/

It adds async features to php.

After that its trivial to add mysql and redis connection pooling:
https://github.com/open-smf/connection-pool

Some PHP frameworks come with pooling built-in: https://hyperf.wiki/2.2/#/en/pool