Mail quits working after upgrade

I upgraded my Mac to OS 10.4, which required a re-install of PHP (5.0.4, from Marc Linayage's site on entropy.ch). Since then my pre-existing PEAR Mail package no longer works. I get the message Email error Unable to find class for driver smtp when I try to execute locally, even though my source code is identical to a version out on a server (running PHP 4.3.11) where the Mail function works just fine. My first thought was that my new PHP package hadn't included PEAR support, but Liyanage's page says it does, and an execution of phpinfo() shows no "without PEAR" option (indeed no mention of PEAR at all). Is there some kind of reinstall procedure I need to go through to re-establish this function? Is there some kind of line command that will verify that I have PEAR support on my new PHP installation? Thanks for any help.