public function testOrder()
{
$item1 = 'item 1';
$item2 = 'item 2';
$item3 = 'item 3';
$items = [$item1, $item2, $item3];
$stack = new Stack($items);
$this->assertSame($item3, $stack->peek());
$pops = [];
$iters = [];
foreach ($stack as $element) {
$iters[] = $element;
}
while (($item = $stack->pop()) !== null) {
$pops[] = $item;
}
$this->assertSame($iters, $pops);
$stack->clear();
$this->assertNull($stack->peek());
}