PHP script, to protect my script !

Hi, I have a script that i would like to sell to everyone, but i would like a code to protect it. What my intention is : i would like to had a code into each page that will check the site where is the script. If is not the site i have sell, then the script don't work. I tought about something. Let's say i had a php line that refer to a page that check the script, the code check if the site is load on the good site. So if they remove the line that i will encrypt, the script didn'T work and same thing for the php page that check the script. I have another web site that we could use a DB in case we need it. So that mean that someone will be able to make any modification to the script, but without the php you have to created it didn'T work. The purpose behind it is to give flexibility to coder that would like to work on the script, but without the SPECIAL fonction that you will created it won'T work, even if they work on. I'm open to any suggestion, What you created will be encrypted for more protection. What i ask i have see that on another script. Good luck to all, Alain