Dada a classe abaixo, em PHP:...

$cart1.Cart->add_item("10", 2);

$cart1.Cart->add_item(2, "10");

$cart1 = Cart->add_item(2, "10");

$cart1 = new Cart; $this->add_item(2, "10");

$cart1 = new Cart; $this->add_item("10", 2);