PHP 5.3 is a big leap forward for PHP and brings of a lot of neat features. However, big leaps can also mean big changes and potentially big breakage when it comes to backwards compatibility. I did some experimenting with running a big legacy application and a CakePHP application on PHP 5.3 and would like to share my findings with you. Here are a couple of tips to prepare your code for PHP 5.3

He mentions things like:

• Updates to short tag support
• Deprecation warnings
• MySQL driver support
• Extensions

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.