Hide Script Protection in public classes

Is hard nowadays to stop getting your scripts shared everywhere so Im thinking to add some license checking/stopping functions inside public classes like phpmailer or rsspie that my scripts use..This are 50-100Kb files so there will be kinda hard for a nuller to check line by line this files thinking there are public anyway and not modified .. Am I allowed to do that?

What ya think abt this ?cguru:

Yes you can do it, actually quite simple to do. make it use the HTTP_HEADER and email you the domain for which it is attached to. Also can have it email you the key if your program requires a key that is. With a few tactics you can make it harder to crack, make the code bounce a round and have multiple calls of it, one checks the other and if its there, it does nothing...if it isnt...it continues where there other left off.