Just wanted to add a note for anyone looking to use the MySQLi persistent connections feature; it's important to note that PHP opens and retains one connection per database user per process.
What this means is that if you are hosting multiple applications, each with its own database user (as is good practice) then you will end up multiplying the number of connections that PHP may hold open.
For example, if you have PHP configured with a maximum of eight worker processes, and you regularly use four different database users, then your MySQL server will need to accept at LEAST a maximum of 32 connections, or else it will run out.
However, if you would like to minimise the number of connections, what you can do is instead is to open the connection using a "guest" user (with no privileges except logging in) and then use ->change_user() to switch to a more privileged user, before switching back to the guest when you're done. Since all of the connections would therefore belong to the guest user, PHP should only maintain one per worker process.
