Not quite sure about exactly how PHP handles persistent connections. Knowing a bit of the technology behind it all, I would guess that it would operate exactly the way you'd expect it to via a web browser. ie. Before opening a new db connection, the script would search and see if a socket has already been opened.
This is a sidenote, but I'm curious what you're up to with Asterisk? My two cards from Digium should arrive either today or tomorrow (shipped via slow boat) and I can't wait to see what I can put together. I've been to the link you mentioned and have a ton of ideas swirling around for using PHP and Asterisk.
Maybe we could work together and get the first PHP/Asterisk open source project started, now that would be cool.