Either PHP wasn't compiled successfully or the module isn't being loaded (for module installs) ...
Check your httpd.conf file for a "LoadModule" line for PHP and make sure it is NOT commented out!
If you are using SuPHP or phpSuExec then disregard what I just said and it's probably more likely
that PHP didn't compile succesfully and for that I would recommend a manual install of PHP 5 as
opposed to using EasyApache or EasyPHP because those are not so "Easy".