How can k encode a copyright link so people can't remove it and if they do the script doesn't work properlly?
Make every depend on the string. For example, encrypt everything at the database with a crc of that string ..omg its friday
You could try ioncube or zend guard. Realistically, even with these you can't do it with php. Just make a good product and develop a good and fair usage license. Honest users will adhere to it, and the dishonest ones would have bypassed your protection anyways. Requiring links completely unrelated to a programming/design service or the theme itself is spam which is why most people hack their templates.