For those people having problems with persistent connections to MySQL under big loads, this might help them. If they are using Linux, there is a problem with MySQL for Linux and multithreading. This process is explained in page 72 for the 3.23.33 version. Thus if apache has a max of 200 children, there is going to be a max of 200 connections to mysql, even if only 10 are used at any one time. The reason the php people probably did it this way is that if done any other way it would be a lot more complicated to make sure connections aren't used on multiple childs at once.
Worse things can happen with data changing in the middle of processing results and such things.
Of course, a quick workaround is selecting the database before each query. I fails to take over an unused sleeping thread. Every pconnect command opens its own thread in the mysql server. I encountered a problem, that mysql opens up a new connection to the database although I used it. No se que inconvenientes puede tener.