You could just create a licence file that is installed along with the product, and is tied to the domain. If you obfuscate the PHP code (I'm assuming the product is PHP) and do a simple encryption on the licence file, it would be difficult for a novice to crack.

For what it's worth, ioncube is the most difficult of php protections to crack. Even seasoned hackers/crackers have difficulty with it. I use it to encrypt products and can restrict to domain/ip or even mac address of the server's network card.
Support team is excellent, product is well worth the investment.

I'll second that. IonCube is what you're looking for. It might not be the cheapest option, but right now it's the best.

The only problem you'll have with it is if someone decides to try installing your product on a Windows-based server. The web host will have to install the IonCube loaders into the server since most Windows servers won't properly handle "dynamic loading" the same way most Linux servers will.

Thanks for your replies.
Mike