$GLOBALS

We can be more clear with the extension of the given example.

Before PHP Version 8.1

<?php
  $a = 1;
  $globals = $GLOBALS;
  $globals['a'] = 2;
  echo $a; // 2
  echo $globals['a']; // 2
  echo $GLOBALS['a'];  // 2
?>

After PHP Version 8.1

<?php
  $a = 1;
  $globals = $GLOBALS;
  $globals['a'] = 2;
  echo $a; // 1
  echo $globals['a']; // 2
  echo $GLOBALS['a'];  // 1
?>

Watch out when you are trying to set $GLOBALS to the local variable.

Even without reference operator "&" your variable seems to be referenced to the $GLOBALS

You can test this behaviour using below code

<?php
/**
 * Result:
 * POST: B, Variable: C
 * GLOBALS: C, Variable: C
 */
 
// Testing $_POST
$_POST['A'] = 'B';
 
$nonReferencedPostVar = $_POST;
$nonReferencedPostVar['A'] = 'C';
 
echo 'POST: '.$_POST['A'].', Variable: '.$nonReferencedPostVar['A']."\n\n";
 
// Testing Globals
$GLOBALS['A'] = 'B';
 
$nonReferencedGlobalsVar = $GLOBALS;
$nonReferencedGlobalsVar['A'] = 'C';
 
echo 'GLOBALS: '.$GLOBALS['A'].', Variable: '.$nonReferencedGlobalsVar['A']."\n\n";

I finally found information about superglobals not being found in $GLOBALS:

https://bugs.php.net/bug.php?id=65223&edit=2
-------------------------------------
 [2013-07-09 12:00 UTC] johannes @php.net
[...]super-globals (aka. auto globals) are not added to symbol tables by default for performance reasons unless the parser sees need. i.e. 

<?php
$_SERVER;
print_r($GLOBALS);
?>

will list it. You can also control this using auto_gloals_jit in php.ini: http://www.php.net/manual/en/ini.core.php#ini.auto-globals-jit
-------------------------------------

http://www.php.net/manual/en/language.variables.variable.php
-------------------------------------
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.
-------------------------------------

As of PHP 5.4 $GLOBALS is now initialized just-in-time. This means there now is an advantage to not use the $GLOBALS variable as you can avoid the overhead of initializing it. How much of an advantage that is I'm not sure, but I've never liked $GLOBALS much anyways.