In a recent post to the Pantheon blog community member Cal Evans makes one request for the developers out there (PHP and others) - please license your code.
It is wonderful that you have put your code up on GitHub. That is the essence of "Social Coding". However, if you do not put a license on it, you are just teasing developers. In essence, you are saying "See what I made? You can't use it, but I wanted to show you anyhow." Granted, sometimes, developers will use unlicensed code in their projects anyhow, but usually not. Without a proper license, others have no idea what is a permissible use. You wrote it, you own it, you shared it, so let people know they can use it.
He points out that GitHub makes it easy to just throw code up and expose it to the word for use. Unfortunately, due to restrictions put in place by business or technology groups, code without a license simply can't be used. If you're not familiar with code licenses, he links to the Choose a License site that can walk you through the choice via a series of questions.