Added Deck.php which defines new Deck class - represents a physical deck of cards, with each 'card' being arbitrary data loaded from JSON files used to define decks (so a card can be of any type which JSON supports).
It is also possible (and probably often desirable) to easily subclass Deck so cards can be converted from their serialised form into any datatype by simply overriding the loadCard() (& possibly saveCard()) method in the subclass (e.g. the card could be stored in JSON as a UID & converted to a corresponding object when the Deck is loaded)

N.b. Should probably add a JSON Schema (at the very least) to explain/define the correct format for Deck JSON files