what does "nulled script" mean?

I'm learning about web development and I just came across the term "nulled script". I googled it, but I still don't get it. What does it mean? I don't really understand how open source php, javascript web 开发者_StackOverflow社区software in a LAMP environment can be protected or at least how the software can call home if it's modified or transfered to a new server... I would be great if somebody could explain it in a few words. Thanks. (Just to make it clear: I don't want to steal code. I am interested in protecting my own code.)

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A nulled script is when somebody changes a script to remove the protection implemented by the author. For example, when they remove call homes or registration checks or etc. These scripts are commonly distributed by shady/warez sites.

Although nearly impossible to detect from original author, I have no idea of the legal implications from removing call-home code in a script, javascript code has to be accessible to be of any use and is not compiled. As for removing copyrights and licensing information in a script, (also very hard to detect) this is pretty much illegal as far as I know.