I'd be curious to hear about experiences with persistent pooling usage within PHP, since I don't feel that I've heard about it being advised as much myself over the years, but that could perhaps be due to old misperceptions about it.
I know there is the "Persistent Database Connections" section of the PHP manual and the mysqli extension within PHP supports connection pooling / persistent connections, but in my own experiences I've rarely seen them utilized, especially by the bigger open source projects out there such as WordPress, which has an 8 year old enhancement topic on the subject: https://core.trac.wordpress.org/ticket/31018. Putting your database behind a pooler, like ProxySQL let's say, is another option as the level of sophistication for a company/application increases, but most typical PHP setups I've used don't have that immediately available.
I've generally been under the impression that most projects/applications don't use the built-in pooling features for some of the reasons discussed in the link above, leading to those applications being more impacted by lengthier connection times due to a new connection being created at the beginning each request and then closed at the end of the request.
Now I'm inclined to experiment a bit with the built-in mysqli pooling feature though since it would seem a worthwhile feature for developers to experiment with more if it would lessen the connection time impact for each PHP request, particularly for databases that are further away and require secure connections.
Shaving off 100ms for a connection would be significant for most PHP users if they are currently having to open fresh connections on each request, especially if they were previously used to connection times of < 1ms when connecting to a local MySQL database.