Protect neighboring sites from the site. How?

Question

linux

There is nginx+phpfpm
There is a directory /var/www
In it:
site1.ru
site2.ru
site3.ru
All three sites are mine and no one except me works with them.
A friend asked me to host his site:
site4.ru
on my server, but
this script works with a bang site4.ru/index.php:

<?php
$fp = fopen("../../site1.ru/htdocs/test.txt", "w");
fwrite($fp, $text);
fclose($fp);
?>

In the end, I'm worried)
How to be? So that he can execute and read files no further than his root directory.

Answer 1

отchrootи его - https://gir.me.uk/nginx-php-fpm-with-chroot/

Answer 2

Надо каждому сайту своего пользователя делать и свою папку

Answer 3

chroot + basedir