[eluser]Georgi Budinov[/eluser]
Well there are software products that have this feature - some of which are paid and expensive.
IonCube is one of them and it is the most popular - well paid also. It obscures the code in byte cde as I know, makes the execution faster and adds features like the one you are looking + code working on domain basis.
ZendGuard is the other solution - pretty much the same but I don't know much details on it. You can look at it.
Of course there are others cheaper and free solutions which are of course easier to brake .. hack ..
I have heard of breaking ionCube too, One day I tried some how-tos on how one can do that .. no success - perhaps the reason is jjust me