What is object-oriented programming?

An object is a thing with rules

An array can contain anything. An object contains exactly what its class provides for, and it makes sure itself that it stays in a valid state. That is the whole difference — and it is a big one.

// As an array: nothing stops you storing nonsense
$urlaub = ['von' => '2027-08-01', 'bis' => '2027-07-01'];
// End before start — only noticed somewhere further down the line

// As an object: the case cannot arise at all
final class Urlaubszeitraum
{
    public function __construct(
        public readonly DateTimeImmutable $von,
        public readonly DateTimeImmutable $bis,
    ) {
        if ($bis < $von) {
            throw new InvalidArgumentException('Ende liegt vor dem Beginn.');
        }
    }
}

Encapsulation: only what is needed on the outside

Encapsulation means that an object’s data cannot be changed from outside, only through methods that know the rules. In practice this means: when a rule changes, you change it in one place instead of hunting through all the callers.

The counter-test is simple: can an object be brought from outside into a state that the business rules say must not exist? Then encapsulation is missing.

Interfaces: the most important part

An interface — in the sense of the language, not of data transfer — defines what an object must be able to do, without saying how. That sounds abstract and is the most practical part of the whole idea.

interface Benachrichtigung
{
    public function sende(string $empfaenger, string $text): bool;
}

final class EMailBenachrichtigung implements Benachrichtigung
{
    public function sende(string $empfaenger, string $text): bool
    {
        // ... actual sending
        return true;
    }
}

// The caller only knows the interface:
function informiere(Benachrichtigung $weg, string $an): void
{
    $weg->sende($an, 'Ihr Auftrag wurde angenommen.');
}

That is exactly what interfaces are for: they separate the decision “what should happen” from the decision “by what means”. In a test, the same place can be filled with a simple stand-in that sends nothing but records what it received.