typehint.php

function addition(NumbersInterface $numbers) {
    // We know for sure that we can access $numbers->number1, $numbers->number2
    // and $numbers->number2. This function can now do its one thing without
    // being burdened with validation.
    // http://en.wikipedia.org/wiki/Single_responsibility_principle
    // http://en.wikipedia.org/wiki/Design_by_contract
}

interface NumbersInterface {
    public $number1;
    public $number2;
    public $number3;
}

class Number implements fooInterface {
    public $number1 = 1;
    public $number2 = 2;
    public $number3 = 3;
}