On the PHP-GTK Community site, there's a new tutorial showing how to install the PHP-GTK platform to a Mac OS X machine:

Installation of PHP-GTK 2 on MacOS X 10.4 and 10.5 is actually pretty simple, as it can use the latest PHP-GTK installer, which works just like any other Mac installer.

They include screenshots as a guide to the step-by-step (extremely simple) process of getting the software installed.

For those interested in technical details, a complete list of the prerequisites is given at step 2 of the installation. Download the installer at http://www.kix.in/php-gtk2/PHP-GTK-2.0.0.dmg