If you've been puzzling on how to get a custom installation of PHP, MySQL and Apache set up on your Snow Leopard (OS X) upgrade, Rob Allen has just the post you've been looking for:
With OS X 10.6, Apple ships PHP 5.3 with PEAR, GD and PDO_MYSQL out of the box. Also, everything is now 64bit. This means that the entire effort required to get a working PHP dev environment for my work is now much easier.
He walks you through the modifications needed for files in /usr/local, changes to get MySQL started, updates to the Apache configuration to get it playing nicely with the PHP5 module and changes to the php.ini and Xdebug configurations to pick up the right extensions directory.