PDO MySQL extension

Thanks in advance for helping. I am a Yii newbie and this is my first post. My problem is the Yii Requirements Checker indicates a PDO MySQL extension warning.

I’m running OS X 10.5.8 on a Mac PowerPC while awaiting Apple to update their computer line. Apache/2.2.11 (Unix) and PHP 5.2.8 seem operational. MySQL 5.0.67-osx10.4-powerpc is the best my setup will accommodate. MySQL starts when the computer boots. I’ve looked through /etc/php.ini. What have I missed?