I read all of it down to 1.4 due to the fact that the html ends with a .php such as http://www.interpals.net/login.php and clicked all around to find an id= but could not. I may sound stupid for saying this but I am just wanting to learn even it does sound that way because some times you just have to go in head first. But I was wondering if this is till SQL injectable and not the others that you listed like RFI or LFI. Help and input would be greatly appreciated.
You can look for more than id=, any argument will do. And yes, it can still be vulnerable to SQL injection even tho no result is found by following that tutorial. There are a variety of other ways a page can be vulnerable to SQL injection. Among them is POST arguments and other HTTP header fields. But that is slightly more advance.
EDIT: Also, the file extensions doesn't have to be .php, can be anything.