这段PHP代码是什么意思呀？

$globals = array($_REQUEST, $_SESSION, $_SERVER, $_FILES);

foreach($globals as $global){
    foreach(array_keys($global) as $key){
        unset(${$key});
    }
}

$a = 1;
$b = 2;
$c = 3;
foreach(array('a', 'b', 'c') as $k) echo ${$k};