$GLOBALS

This super-global is a PHP predefined variable, which holds a reference to every global variable in the application.

<?php

// implicit global variable
$aGlobalVariable = 1;

function foo() {
    echo $GLOBALS['aGlobalVariable'];
    $GLOBALS['aGlobalVariable'] = 2;
}

foo(); // displays 1
echo aGlobalVariable; // 2

?>

Documentation