Interfaces

interface Vehicle {
    public function whatAreYou();
}

class Plane implements Vehicle {
    public function whatAreYou() {
        print "I'm a plane.";
    }
}

class Bicycle implements Vehicle {
    public function whatAreYou() {
        print "I'm a bicycle.";
    }
}

// Declare $vehicle of the type "Vehicle". It can now hold any derivative of the "Vehicle" interface.
Vehicle $vehicle;

// This is acceptable:
$vehicle = new Bicycle();

// This works too!
$vehicle = new Plane();

// But really none of this matters, since a PHP variable can hold anything anyway. but with languages as C++ or Java, a variable always has a type, so even a boolean wouldn't fit in an integer. Nor a float in a boolean (etc).