Legality of software development

So I am a college student, and at my job I made a php website that would be beneficial to where I work. Recently, they have asked me for it. The thing is, programming is definitely NOT part of my job description, and my friend suggested that I should not just give it to them, possibly sell it to them.
I'm not really up with the legality of all this, was wondering if anyone knew.

So I am a college student, and [b]at my job[b] I made a php website that would be beneficial to where I work.

It's theirs.