Superglobals can't be accessed via variable variables in a function?

Question

I can't access superglobals via variable variables inside a function. Am I the source of the problem or is it one of PHP's subtleties? And how to bypass it?

print_r(${'_GET'});

works fine

$g_var = '_GET';
print_r(${$g_var});

Gives me a Notice: Undefined variable: _GET

Answer 1

PHP isn't able to recognize that this is a global variable access:
It compiles $_GET and ${'_GET'} to the same opcode sequence, namely a global FETCH_R. ${$g_var} on the other hand will result in a local FETCH_R.

This is also mentioned in the docs:

Superglobals cannot be used as variable variables inside functions or class methods.