PHP Sort Order ASC and DESC

I have the following code:

$sort = (isset($_GET['sort'])) ? $_GET['sort'] : 'cn';

// Determine the sorting order:
switch ($sort) {
	case 'cn':
		$order_by = 'card_number ASC';

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When implemented it works fine.  However, how do I add to that DESC? So, If I click on the link it orders everything in an ASC manner and when I click on it again it sorts in a DESC manner?