Interfaces & Abstract Classes

<?php
interface Payable {
    public function pay(float $amount): bool;
    public function getBalance(): float;
}

interface Notifiable {
    public function notify(string $message): void;
}

// Class must implement ALL interface methods
class CreditCard implements Payable, Notifiable {
    private float $balance = 5000;

    public function pay(float $amount): bool {
        if ($amount > $this->balance) return false;
        $this->balance -= $amount;
        return true;
    }

    public function getBalance(): float {
        return $this->balance;
    }

    public function notify(string $message): void {
        echo "SMS: $message\n";
    }
}

$card = new CreditCard();
$card->pay(100);
echo $card->getBalance();  // 4900
$card->notify("Payment of ¥100 received");
?>

<?php
abstract class Report {
    // Abstract method — child MUST implement this
    abstract protected function getData(): array;

    // Concrete method — shared by all children
    public function generate(): void {
        $data = $this->getData();
        echo "<h1>" . $this->getTitle() . "</h1>\n";
        foreach ($data as $row) {
            echo "- $row\n";
        }
    }

    abstract protected function getTitle(): string;
}

class SalesReport extends Report {
    protected function getData(): array {
        return ["Jan: ¥100,000", "Feb: ¥120,000", "Mar: ¥95,000"];
    }

    protected function getTitle(): string {
        return "Monthly Sales Report";
    }
}

$report = new SalesReport();
$report->generate();
// <h1>Monthly Sales Report</h1>
// - Jan: ¥100,000
// - Feb: ¥120,000
// - Mar: ¥95,000
?>