Fatal error: Declaration of .. must be compatible with .. PHP

Question

I\'m getting the following error:

Fatal error: Declaration of Shoppingcart::addToCart() must be compatible with that of Ishoppingcart::addToCart() in klassen         


        

Answer 1

The declaration of a public function in a sub class must match that of its parent:

public function addToCart();
public function addToCart( Product $product)

You cannot add a parameter to the signature.

This is closely related to the Liskov substitution principle.