What's to stop anyone just removing the include call? To be fair, it is a very poor "solution" if one can call it such.

As for better alternatives, there a number of technologies out there which might help but I think you might be approaching the problem wrongly. If you really, really don't want people to be able to edit the source then make it a hosted solution. If that is not possible, then you'll have to jump through hoops (think Zend encoder, ioncube and such like) or just submit to the open-source philosophy and ask that the links remain in the source.