Abstrakte klasser og metoder

<?php
abstract class Animal {
    public function __construct(protected string $name) {}

    // Must be implemented by each subclass
    abstract public function sound(): string;

    // Shared concrete method
    public function introduce(): string {
        return "I am {$this->name} and I say: " . $this->sound();
    }
}

<?php
class Dog extends Animal {
    public function sound(): string { return 'Woof'; }
}

class Cat extends Animal {
    public function sound(): string { return 'Meow'; }
}

$animals = [new Dog('Rex'), new Cat('Whiskers')];
foreach ($animals as $a) {
    echo $a->introduce() . PHP_EOL;
}
// I am Rex and I say: Woof
// I am Whiskers and I say: Meow

<?php
abstract class DataProcessor {
    // Template method: defines the algorithm skeleton
    final public function process(array $data): array {
        $data = $this->validate($data);
        $data = $this->transform($data);
        return $this->format($data);
    }

    abstract protected function validate(array $data): array;
    abstract protected function transform(array $data): array;

    protected function format(array $data): array { return $data; }
}

<?php
// Use abstract class when:
// - Subclasses share implementation code
// - You need constructor logic or state

// Use interface when:
// - Defining a contract (no implementation needed)
// - Multiple inheritance of type is needed

// A class can only extend ONE abstract class
// but can implement MULTIPLE interfaces

<?php
abstract class Repository {
    // Concrete method available to all subclasses
    protected function findById(int $id, string $table): array {
        $stmt = $this->getDb()->prepare("SELECT * FROM $table WHERE id = ?");
        $stmt->execute([$id]);
        return $stmt->fetch(PDO::FETCH_ASSOC) ?: [];
    }

    abstract protected function getDb(): PDO;
    abstract public function find(int $id): array;
}