how to make sure a link cannot be removed from source code?

wvb it is practically (if not) impossible to get the php back once its encoded with the likes of ioncube. Having a license will also not stop them from removing the links if they really want to.

buddhi225 don't explained it but I supose he provides full websites (images, CSS and source code). Maybe it's not the case so it's a supposition.

In this case, if he provides websites with encoded source code, his websites will be hard to customize or use (if there is no some type of admin panel).

If all the code is modifiable except the links part, their customers can detect this part and delete it.

If all the code is modifiable and he uses include() or other function, their customers also can delete this part.

For these reasons I think that a license can be useful.