license key for php script

I have a script, and I sell it to some people. I need a way to make sure that my script won't work on any website that not in my clients list.

First of all, I am using IonCube to encrypt my PHP code.
I have all my clients in my server database.
Each Client has a domain name, email, name, phone.

What is the best method to avoid making people from stealing my script?

I read about making a license key in the script, so any script without license won't work. However, there are many ways to generate a license key without taking my permission right?

All I need is to not activate any script in any domain name, unless I have it in my clients list.

Solution

There's a reason Adobe, Microsoft, and others don't over actively pursue pirates (not saying they don't, just not at epic, absolutism levels) - they make most of their money from business to business sales and support. A simple license and support structure is typically enough to posture yourself for profit from legitimate businesses and parties who want your product.

Technical protection is a losing battle if you're going to give anyone the code. That's why SaaS is so popular.