phpunit testing methods with $this values - unit-testing

Question

OK, probably a bad example. Here is an actual example. I would be wanting to unit test the total() method from a shopping cart class to see if the value is what I think it should be, which uses the getAll() (cart items), subTotal() and shipping() methods. Is total() something you could test using PHPUnit ?
class Cart extends ObjectModel
{
protected $uniqueID = 10;
public function getAll()
{
return $this->execute("SELECT *, cart.id AS cart_id FROM cart LEFT JOIN products ON products.id = cart.product_id WHERE cart.unique_id = ? AND cart.quantity > '0' AND cart.deleted_at IS NULL ", [$this->uniqueID] );
}
public function subTotal()
{
$subTotal = 0;
foreach($this->getAll() as $row){
$subTotal += ( $row->quantity * $row->cart_price );
}
return $subTotal;
}
public function total()
{
return $this->subTotal() + $this->shipping();
}
public function shipping()
{
if(isset($_SESSION[SALT.'shipping']) && $_SESSION[SALT.'shipping'] != ''){
return $_SESSION[SALT.'shipping'];
}
/* OR RETURN DEFAULT IF SHIPPING IS NOT SET */
return 0;
}
}
The execute method is as follows, there is no constructor method for the ObjectModel class
public function execute($query, array $array) {
$query = Db::conn()->prepare($query);
$query->execute($array);
return $query->fetchAll(PDO::FETCH_OBJ);
}
Db::conn() is just the PDO database handle.

Answer 1

The class is not too well designed for unit-testing because it is tightly coupled to the database. In a unit-test, you want to test the subject-under-test in isolation of external components (here: the database). Usually, you mock or stub the external components with a test double.
Because the DB instance is hardcoded in the execute method of the ObjectModel, you cannot easily replace it with a test double. You could turn the test into an integration test and change the DB connection to a test database which you then populate with expected values in the tests setup method.
If you don't want to use the database at all, you can self-shunt the subject-under-test. This means, you use a test double of the subject-under-test itself and make execute return the expected values for your test case.