When a user accesses a server through a browser, which of the users on the server is the master?

Who is the user on the server when the user accesses the server through a browser?
RT, I mean, when the server is running, for example, at the same time there are 5 users logged on the server, its users who are logged in?

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Thank you for your help!
In fact, I also feel that the problem is not well described, but I do not know how to describe ...

For example, in the PHP page there is such a sentence: mkdir (' Test ');
Who is the user and user group of the test folder?

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