extends

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# `extends`

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 substract 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'.

    class Named_Cart extends Cart
    {
        var $owner;

        function set_owner ($name)
        {
            $this->owner = $name;
        }
    }

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:

    $ncart = new Named_Cart;    // Create a named cart
    $ncart->set_owner("kris");  // Name that cart
    print $ncart->owner;        // print the cart owners name
    $ncart->add_item("10", 1);  // (inherited functionality from cart)

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.

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