Polymorphism is an OOP feature that allows us to work with different classes that implement the same interface. It is one of the beauties of object-oriented programming. It allows the developer to create a complex system of classes and hierarchic trees, but offers a simple way of working with them.

Imagine that we have a function that, given a payer, checks whether it is exempt of taxes or not, and makes it pay some amount of money. This piece of code does not really mind if the payer is a customer, a librarian, or someone who has nothing to do with the bookstore. The only thing that it cares about is that the payer has the ability to pay. The function could be as follows:

function processPayment(Payer $payer, float $amount) {
    if ($payer->isExtentOfTaxes()) {
        echo "What a lucky one...";
    } else {
        $amount *= 1.16;
    }
    $payer->pay($amount);
}

You could send basic or premium customers to this function, and the behavior will be different. But, as both implement the Payer interface, both objects provided are valid types, and both are capable of performing the actions needed.

The checkIfValid function takes a customer and a list of books. We already saw that sending any kind of customer makes the function work as expected. But what happens if we send an object of the class Librarian, which extends from Payer? As Payer does not know about Customer (it is rather the other way around), the function will complain as the type hinting is not accomplished.

One useful feature that comes with PHP is the ability to check whether an object is an instance of a specific class or interface. The way to use it is to specify the variable followed by the keyword instanceof and the name of the class or interface. It returns a Boolean, which is true if the object is from a class that extends or implements the specified one, or false otherwise. Let's see some examples:

$basic = new Basic(1, "name", "surname", "email");
$premium = new Premium(2, "name", "surname", "email");
var_dump($basic instanceof Basic); // true
var_dump($basic instanceof Premium); // false
var_dump($premium instanceof Basic); // false
var_dump($premium instanceof Premium); // true
var_dump($basic instanceof Customer); // true
var_dump($basic instanceof Person); // true
var_dump($basic instanceof Payer); // true

Remember to add all the use statements for each of the class or interface, otherwise PHP will understand that the specified class name is inside the namespace of the file.