what is Variable variables

Answer:

A variable variable takes the value of a variable and treats that as the name of a variable

The above may be little bit confusing

See the example

//declare a variable

$a='allphpinterviewquestions';

variable of variable means

$$a = 'test';

now $allphpinterviewquestions value is test

How to unset a reference

Answer

Use unset

$a = 1;
$b =& $a;
unset($a);

This will not unset $b;

List of Predefined Variables in php

Answer

======

$GLOBALS

$_SERVER

$_GET

$_POST

$_FILES

$_REQUEST

$_SESSION

$_ENV

$_COOKIE

$php_errormsg

$HTTP_RAW_POST_DATA

$http_response_header

$argc

$argv