Matthew Turland has posted a guide he created following his own process as he worked to compile PHP 5.3.0RC1 with PHP-GTK/Cairo support on a Ubuntu (Jaunty) installation.

The process was a bit arduous, as Ubuntu apparently has a rather "interesting" automake package, so I thought I'd document it here for anyone who might be interested in repeating the process. [...] I'm assuming here that you want to use as many available Ubuntu packages as is feasible, aside from maybe PHP itself, in order to minimize the amount of manual compilation necessary.

He works through the steps that include installing several packages (via apt-get), compiling PHP to work with PHP-GTK, compiling and phpize-ing the Cairo extension and including that into the php.ini for the local installation. There's even a quick test script (a clock) you can run to see if all it working correctly.