is_readable What the user name is when deciding what to do with the

What is the user name when the is_readable is judged
BOOL Is_readable (string filename)
Returns TRUE if the file or directory specified by filename exists and is readable.
Remember: PHP can only access files with the user name (usually ' nobody ') that runs webserver. Restrictions that do not count toward Safe mode.

This function determines the user name to know, where can this user name know?
------to solve the idea----------------------
It's Apache's user name, usually www-data.

Open/etc/apache2/apache2.conf
Find the user and group.

------to solve the idea----------------------
There are some _www upstairs, some of them are Apache2.

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