To the above poster, if you misunderstood what he was asking, fair enough, else try to learn what include actually does

Well first of all you'll need to make sure you explicitly agree this with every single client you do (common sense, but just wanted to be clear), and secondly this is a point of contention amongst programmers.

There's several ways of doing it - trust them not to remove it being the easiest answer, or write some very complex code to decode an obfuscated base64 encoded string (see base64_decode & base64_encode on php.net) being the most common ones. You'd want to do something in that code that couldn't easily be replicated - something key to the script that would be hard to replace if removed.

In the end though, is it really necessary? Any obfuscation is both slow & shows a severe lack of trust - and anyone could easily hire a $1 an hour programmer from India etc who would be able to break it in the end no matter what, costing the client only $10-15 to remove your link.

Not to mention that if this is for client work then most clients who needed to subcontract work wouldn't even consider you if you followed this policy, and many of your normal clients wouldn't be happy either unless you offered them a reasonable discount.

EDIT: Due to this post further down the thread, I henceforth apologise for not referring to wvb as wvb and instead referring to him by the perfectly factually correct statement of 'the above poster'. This was never intended to bruise his apparently enormous ego or point out that include alone would still not solve the problem here.