We usually develop on Linux and Mac OS X and those systems are very well tested. But some people are also using Okapi on Windows.
We recommend the latest and greatest PHP, which currently is PHP 5.3.x.
Following Extensions are recommended (and enabled by default in a standard PHP installation):
(they are not all really needed for the core Okapi, but are usually expected to be installed)
Depending on what you develop, which templating system and DB you use, the following may also be needed. This tutorial needs them.
Further useful extensions
We recommend using Apache 2.2/2.0, but 1.3 should still work.
mod_rewrite is essential for making Okapi work.
We usually install PHP as module (mod_php), but fastcgi should also work.
Depending on your application you want to write, you may need a Database. For this tutorial I used MySQL 5.0.x.
Last but not least, you need a webbrowser. For Okapi itself, there are no special requirements, but as we usually use YUI or similar libraries, we recommend using an A-Grade browser from the YUI Browser Support Chart
For a serious developement setup (and to follow this tutorial), you also need a subversion client. You can use a GUI based one, but to better learn what's going on, we recommend using the command line for this tutorial.