It has been made very simple for us.

Install the package 'eclipse' through a package manager. Make sure eclipse-jdt and eclipse-pde are also installed (without them installing PDT will not work).

Then start Eclipse, choose a Workspace directory and then goto the menu:

Help > Install new software > add...

Name: Official
Location: http://download.eclipse.org/releases/galileo/

Wait a minute while it is loading a list of all plugins.

Under "Programming languages" there is PHP Development Tools!

Select and install!

Here ya go :)