With an interpreted (not compiled) language like PHP it's difficult to prevent source code from spreading once it's been released. The encoding solutions provided by Macik attempt to obfuscate the source code so it becomes difficult to modify it. Then you can use some sort of serial code system to limit use.
My suggestion is to use a legal license which indicates the limited amount of domains your package may be used on. Some won't abide it, but I doubt it's worth the effort to use an encoder.