Encyrpt Get Variables

You have a link on your page to test.php?course=5&id=68&d=ff. Some of the code on that page is manipulated by the variables in the URL string. Is there a way to encrypt/decrypt the variables in the URL? Like test.php?fi=sa4566w87as5? Is there a term for this? I could probably figure it out if I knew the term.

I know I can create checkpoints to make sure the variables in the string are reasonable, but that can be quite cumbersome. I just need a good way to encrypt/decrypt them on the fly.