how will i do this?

hello all!!! i was wondering if i can do a scrip, and when i finish the scrip i need a protection like if it was installed on other web server beasided the one it is licended to work. i thought in doing this by writing in a text file somewhere in the server have just a caracter and then making the scrip check for that file... if the file exist its ok, if not it will send me a email telling me something about the place where it is running and writing the file on the comp (so i just got 1 mail for each server). the problem is that as all this is written in php, the root user can get to the code and disable this... is there any way to round this problem? i thought in enconding this function but does anyone has experience on this thank you all in advance