You need to update the demo scripts. The line
$purchase_request = new FatZebra\PurchaseRequest($_POST['amount'], $_POST['reference'], $_POST['name'], $_POST['card_number'], $_POST['card_expiry_month'] ."/". $_POST['card_expiry_year'], $_POST['card_cvv'], null, 'AUD');
needs to become
$response=$gateway->purchase($_POST['amount'], $_POST['reference'], $_POST['name'], $_POST['card_number'], $_POST['card_expiry_month'] ."/". $_POST['card_expiry_year'], $_POST['card_cvv'], null, 'AUD');
since you've deleted the PurchaseRequest class in FatZebra.class.php