You need to consider your objectives:
1) Are you trying to prevent people from reading/modifying your code? If yes, you'll need an obfuscation/encryption tool. I've used Zend Guard with good success.
2) Are you trying to prevent unauthorized redistribution of your code?? A EULA/proprietary license will give you the legal power to prevent that, but won't actually stop it. An key/activation scheme will allow you to actively monitor usage, but can be removed unless you also encrypt your code. Zend Gaurd also has capabilities to lock a particular script to a particular customer machine and/or create time limited versions of the code if that's what you want to do.SEO company Chicago