PHP 4 is running as the default version for all of ANC's hosted sites. To run PHP 5, simply follow the directions found here:
http://www.anetworkcompany.net/kb/entry/10/
It took me awhile to configure this for my site because I use some fancy custom errors pages that I could only setup in .htaccess, but if you don't need any hefty apache config in .htaccess, then the above solution is quick and painless...
