Installing Drupal 7 on Mac OS X 10.5 (Leopard)
If you are using the standard installation of PHP and MySQL on Mac OS X 10.5 (Leopard), and you want to try out the latest Drupal 7 code, you will run into some problems. The standard installation of PHP that Apple include with Leopard does not include support for MySQL databases through the PDO layer, which Drupal now relies upon to gather information about which databases are available on the system.
There are some suggestions online (see below) for how to add this support to your installation, but your luck may vary. As such, you may find it better to switch to a pre-packaged solution such as MAMP (recent version with PHP 5.2.x or later for Drupal 7).