In order to run Phork you will need PHP 5. Phork will not work with PHP 4.
Phork was developed on Apache but it has been known to run on Microsoft's IIS. It doesn't require mod rewrite (or equivalent) but it is highly recommended to use it for nicer looking URLs.
You will most likely need a database unless you just plan to use Phork to route to static pages. Currently Phork has been written and tested using MySQL but can be extended to use any number of SQL databases.
Phork supports caching via one of 2 Memcache modules or through a Filecache module. Caching is not required, but in order to support Memcache caching your server must have memcached installed as well as either the PHP Memcache or PHP Memcached modules.
PHP's register globals should be turned off.