There is no way to "force' content on an open source script (open source means the source can be modified). There are services like ioncube that will make your PHP code unreadable and therefore unmodifiable, but I'm sure there are ways past that. Otherwise stick to .NET or another compiled language (although .net and any programming language can be reverse engineered).

It comes down to this: Processors do whatever the hell programs tell them to do, and they only accept commands in a uniform way. With that said, it is possible to reverse engineer a program because they are commanding the processor in this uniform manor. So no program is can for certain place anything anywhere if you are dealing with technical people.

edit:
As for WVB's solution, it will not work. As Sal pointed out, there is nothing stopping the user from simply removing the inculde. Any validation would require placing a known value in memory (aka a variable), which can be easily forged on the client's end by even a semi-competent programmer. If a file can be included, the source can be grabbed from it. So embedding a few curtail functions on your side would prove useless.