The website is only for the authorized person to use, does not have the authorization to use, how should realize?

The website is only for the authorized person to use, does not have the authorization to use, how to realize?
For example, the company made a website, other companies want to buy, the company hopes that other companies to buy the only one can use. For example, a, B to buy, then A, b if the program to C or D, but not authorized or to what key so c,d can not be used, how can we solve the implementation?
Remember as if ecshop have such a function, but do not know how to implement it.
@xuzuning Thank you, great God.

------to solve the idea----------------------
Simple point to verify the domain name (different unit domain name is certainly different)
A little more complicated, just verify it through your website.
------to solve the idea----------------------
Method one with PHP extension, more difficult to crack
Method two separates server side and client, cannot crack
Cracking with pure PHP code is a matter of minutes.
------to solve the idea----------------------
Add a code to judge URLs in PHP
And then encrypt all the code.
Only sell the use of the source code not sold
and sell them out.
------to solve the idea----------------------
Program contains authorization judgment, program needs encryption

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