It is easy enough to view the source code for a webpage on the Internet, but how do you look at all the code behind the scenes(php etc)?

You can't. The php (or asp or jsp) code is parsed by a php engine before it ever reaches your browser and all you get is html. It's actually a good thing. If you could see all the scripting code on a site that site would become very insecure and you'd see a lot less useful script online.

The only way to see the code behind the scenes is to have direct access to the files themselve such as through ftp.