// Let's say we need to get the date in our project.

class DateStuff {
    function isItFriday() {
        return (date('w') == 5)
    }
}

// How do you test that? You'd have to run the test on a Friday to get the right input. We can create an abstraction of the date function to get around this:

// The interface that defines how our date class should look
interface IDateManager {
    function getDayOfWeekIndex();
}

// And here's an implementation of that interface
class DateManager implements IDateManager {
    function getDayOfWeekIndex() {
        return date('w');
    }
}

// Now we add the dependency:

class DateStuff {
    private $dateManager;
    function __construct(IDateManager $d) {
        $this->dateManager = $d;
    }
    function isItFriday() {
        return ($this->dateManager->getDayOfWeekIndex()  == 5)
    }
}

// So to use that class, you'd have to instantiate a DateManager and pass it to the DateStuff class when you create it:

$dm = new DateManager();
$d = new DateStuff($dm);
echo $d->isItFriday();

// Now its easier to test this class by creating a deterministic implementation of our date manager:

class DeterministicDateManager implements IDateManager {
    function getDayOfWeekIndex() {
        return 5;
    }
}

// Now we can go:

$dm = new DeterministicDateManager();
$d = new DateStuff($dm);
echo $d->isItFriday(); // Always true