Questions about the PHP fopen () function

PHP fopen () function problem
$DOCUMENT _root=$_server[' Document_root '];

@ $fp = fopen (' $DOCUMENT _root/. /orders/orders.txt ', ' ab ');

The book says-this variable ($DOCUMENT _root) points to the root of the Web server document tree. We use the ".." Represents the parent directory of the document's root directory. For security reasons, is this directory outside the entire document tree? In this example, in addition to the interfaces we provide, we do not want other web interfaces to access it. This path is referred to as a relative path because it describes a file system location relative to the document root directory

I do not understand the place--1. “..” Represents the parent directory of the document root directory? 2. Is this directory outside of the entire document tree?

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.. /is the top-level directory relative to the current file
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Don't understand, wait for the expert to answer
------Solution--------------------
Outside of the document root directory, Apache will not visit, that is, the user is not accessible.

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