http://support.lunarpages.com/knowledge_bases/article/326
As in the KB article above, please make sure that you are in fact running PHP5. I would be willing to bet that your server is one of those servers that runs dual PHP4 and PHP5. You will probably be able to switch to PHP5 in CPanel. If not, then as the last line of the article says, you will need to contact your host to enable PHP5 on your server.