3.13. Interfaces

Class inheritance enables you to describe a parent-child relationship between classes. For example, you might have a base class Shape from which both Square and Circle derive. However, you might often want to add additional "interfaces" to classes, basically meaning additional contracts to which the class must adhere. This is achieved in C++ by using multiple inheritance and deriving from two classes. PHP chose interfaces as an alternative to multiple inheritance, which allows you to specify additional contracts a class must follow. An interface is declared similar to a class but only includes function prototypes (without implementation) and constants. Any class that "implements" this interface automatically has the interface's constants defined and, as the implementing class, needs to supply the function definitions for the interface's function prototypes that are all abstract methods (unless you declare the implementing class as abstract).

To implement an interface, use the following syntax:

class A implements B, C, ... {
    ...
}

Classes that implement an interface have an instanceof (is-a) relationship with the interface; for example, if class A implements interface myInterface, the following results in '$obj is-A myInterface' printing:

$obj = new A();
if ($obj instanceof myInterface) {
    print '$obj is-A myInterface';
}

The following example defines an interface called Loggable, which classes can implement to define what information will be logged by the MyLog() function. Objects of classes that don't implement this interface and are passed to the MyLog() function result in an error message being printed:

interface Loggable {
    function logString();
}

class Person implements Loggable {
    private $name, $address, $idNumber, $age;
    function logString() {
        return "class Person: name = $this->name, ID = $this >idNumber\n";
    }
}

class Product implements Loggable {
    private $name, $price, $expiryDate;
    function logString() {
        return "class Product: name = $this->name, price = $this >price\n";
    }
}

function MyLog($obj) {
    if ($obj instanceof Loggable) {
        print $obj->logString();
    } else {
        print "Error: Object doesn't support Loggable interface\n";
    }
}

$person = new Person();
// ...
$product = new Product();

MyLog($person);
MyLog($product);