Let's look at the whole picture of what we are trying to do and what software we will be using to achieve our aim of debugging Drupal. For simplicity I am going to assume we are going to use a local web server and debug locally on the same machine.
To do this on OS X, the easiest way is to download the Apache 1.3 / PHP 4.3.11 installer package from entropy.ch as the standard version that comes with Tiger does not work.