Where are you seeing this?

That is view-source:https://www.php.net/manual/de/language.variables.predefined.php

in https://www.php.net/manual/en/language.variables.predefined.php it is at the end of the first para:
For a listing of these variables, please see the section on Reserved Predefined Variables.

Oh, it seems to be a translation issue. The engl
en: Reserved Predefined Variables
de: Vordefinierte Variablen ("Reservierte " is missing)

They aren't actually reserved, though. You can assign to and overwrite them if you wish. $_GET, $_POST, even $GLOBALS. It's bad practice, but you *could*.
Additionally the page linked to calls itself "Predefined Variables".

Lets start with the structure of the english version:
https://www.php.net/manual/en/language.variables.predefined.php
links with label "Reserved Predefined Variables" to /manual/en/reserved.variables.php
Both have title "Predefined Variables".

In German we have
https://www.php.net/manual/de/language.variables.predefined.php
linking with label "Vordefinierte Variablen"
to /manual/de/reserved.variables.php
Both have title "Vordefinierte Variablen"

So, 2 different pages with the same title is confusing. 
The first title is OK, the second should be different. 
As newby to PHP I am not entitled to decide, if "Reserved" is the right word or if there exists something better.

In my opinion, there is no problem with multiple pages having the
same title; actually, the manual has a lot of page titled
"Predefined Constants", for example.  The link texts are, however,
unfortunate.