Phpfopen () functions

Phpfopen () function problem $ DOCUMENT_ROOT = $ _ SERVER ['document _ root']; @ $ fp = fopen ('$ DOCUMENT_ROOT /.. /orders/orders.txt ',' AB '); the book says this -- this variable ($ DOCUM php fopen () function problem
$ DOCUMENT_ROOT = $ _ SERVER ['document _ root'];

@ $ Fp = fopen ('$ DOCUMENT_ROOT/../orders/orders.txt', 'AB ');

In the book, this variable ($ DOCUMENT_ROOT) points to the root of the Web server document tree. We use ".." to indicate the parent directory of the document root directory. For security reasons, is this directory located outside the entire document tree? In this example, apart from the interfaces we provide, we do not want other web interfaces to access it. This path is called a relative path because it describes a file system location relative to the document root directory.

What I don't know -- 1. ".." Indicates the parent directory of the document root directory? 2. Is this directory located outside the entire document tree?

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../Is the upper-level directory relative to the current file
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Don't understand. wait for the answer
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Put it outside the root directory of the document, and apache will not access it, that is, the user cannot access it.