Usually "turning on" a additional programming or scripting language like PHP or Ruby do not interfere each other. If you use RadiantCMS and therefore Ruby to generate the contents of your sites you may get in trouble with integrating a PHP program into this environment.
Telling the provider switching on PHP should not cause any trouble with your active installation at all. And there should be a way to execute the PHP somehow - with the correct webserver settings - and get the script running. Just give it a try.
Anyway, if you are not familiar with programming you should have some helping hands. Because what you do when you upload a script to your server is problematic; Everyone can execute it by asking the webserver to do it. This may be an security issue. Please keep that in mind.