While debugging code:
<?php

function foo() {
$GLOBALS['CACHE'] = str_pad('', 1024, 'l') . 'k';
xdebug_break(); // 1
echo 1;
}

foo();
xdebug_break(); // 2
echo 1;

property_value -n $GLOBAL['CACHE'] is available only in global context (xdebug_break(); // 2). The command will be responced by "can not get property" if it was called from function (xdebug_break(); // 1).

Log file is attached.