Unexpected behaviour with variable variables - php

Question

I was trying to pass a variable that contained the name of the superglobal array I wanted a function to process, but I couldn't get it to work, it would just claim the variable in question didn't exist and return null.
I've simplified my test case down to the following code:
function accessSession ($sessName)
{
var_dump ($$sessName);
}
$sessName = '_SERVER';
var_dump ($$sessName);
accessSession ($sessName);
The var_dump outside of the function returns the contents of $_SERVER, as expected. However, the var_dump in the function triggers the error mentioned above.
Adding global $_SERVER to the function didn't make the error go away, but assigning $_SERVER to another variable and making that variable global did work (see below)
function accessSession ($sessName)
{
global $test;
var_dump ($$sessName);
}
$test = $_SERVER;
$sessName = 'test';
var_dump ($$sessName);
accessSession ($sessName);
Is this a PHP bug, or am I just doing something wrong?

Answer 1

PHP: Variable variables - Manual
Warning
Please note that variable variables cannot be used with PHP's Superglobal arrays within functions or class methods. The variable $this is also a special variable that cannot be referenced dynamically.
Solutions
function access_global_v1 ($var) {
global $$var;
var_dump ($$var);
}
function access_global_v2 ($var) {
var_dump ($GLOBALS[$var]);
}
$test = 123;
access_global_v1 ('_SERVER');
access_global_v2 ('test');

Answer 2

From php.net:
Warning
Please note that variable variables cannot be used with PHP's
Superglobal arrays within functions or class methods. The variable
$this is also a special variable that cannot be referenced
dynamically.

Answer 3

The answer is fairly simple: never use variable variables.
Use arrays instead.
(and yes - you are doing something wrong. No, it is not a bug in PHP.)

Answer 4

Use $GLOBALS. There you go :)
<?php
function accessSession ($sessName)
{
var_dump ($GLOBALS[$sessName]);
}
$sessName = '_SERVER';
accessSession ($sessName);