affect of global variable in functon

<?php
function aaa() {
    global $a;
    $a = 2;
    unset($a);
}

$a = 1;
aaa();
var_dump(isset($a) ? 'true' : 'false', @$a);

function bbb() {
    global $b;
    $b = 2;
}

$b = 1;
bbb();
var_dump($b);