Often you need classes with similar variables and functions to another existing class. In fact, it is good practice to define a generic class which can be used in all your projects and adapt this class for the needs of each of your specific projects. To facilitate this, classes can be extensions of other classes. The extended or derived class has all variables and functions of the base class (this is called 'inheritance' despite the fact that nobody died) and what you add in the extended definition. It is not possible to subtract from a class, that is, to undefine any existing functions or variables. An extended class is always dependent on a single base class, that is, multiple inheritance is not supported. Classes are extended using the keyword 'extends'.
This defines a class Named_Cart that has all variables and functions of Cart plus an additional variable $owner and an additional function set_owner(). You create a named cart the usual way and can now set and get the carts owner. You can still use normal cart functions on named carts:
This is also called a "parent-child" relationship. You create a class, parent, and use extends to create a new class based on the parent class: the child class. You can even use this new child class and create another class based on this child class.
Note: Classes must be defined before they are used! If you want the class Named_Cart to extend the class Cart, you will have to define the class Cart first. If you want to create another class called Yellow_named_cart based on the class Named_Cart you have to define Named_Cart first. To make it short: the order in which the classes are defined is important.