GPL License Issues

A project I've been working has come to the attention of a software company that wants to buy what we've done. None of us have much experience in the software business, only development. So we're trying to figure out as much about the business end before we meet with them.

One issue that's come up is that their existing software is probably some kind of MS product, but don't know. We've done everything in Open Source. Will they have any legitimate concerns under the GPL license for software built using PHP/MySQL? My initial response would be no, but honestly I don't know. I have used nothing fancy to develop this, just basic PHP/MySQL.

Is there a website or example on the web that explains the basics?

Thanks much for your help.

K