Inheritance &
Method Overriding

// Parent class — shared shape of all notifications
class Notification
{
    public function __construct(
        protected string $recipient,
        protected string $message
    ) {}

    public function getRecipient(): string { return $this->recipient; }
    public function getMessage(): string   { return $this->message; }

    public function send(): void
    {
        echo "Sending to {$this->recipient}: {$this->message}";
    }
}

// Child class — inherits everything, adds its own detail
class EmailNotification extends Notification
{
    public function __construct(
        string $recipient,
        string $message,
        private string $subject  // email-only property
    ) {
        parent::__construct($recipient, $message); // call parent constructor
    }

    public function getSubject(): string { return $this->subject; }
}

class SmsNotification extends Notification
{
    public function __construct(
        string $recipient,
        string $message,
        private string $phoneNumber
    ) {
        parent::__construct($recipient, $message);
    }
}

$email = new EmailNotification('alice', 'Hello!', 'Welcome aboard');
$email->send();        // inherited from Notification ✓
echo $email->getSubject(); // own method ✓

class Notification
{
    public function __construct(
        protected string $recipient,
        protected string $message
    ) {}

    public function send(): void
    {
        echo "[Base] Notifying {$this->recipient}\n";
    }
}

class EmailNotification extends Notification
{
    public function __construct(
        string $recipient,
        string $message,
        private string $subject
    ) {
        parent::__construct($recipient, $message);
    }

    // Override: completely replace parent behaviour
    public function send(): void
    {
        echo "[Email] To: {$this->recipient} | Subject: {$this->subject}\n";
        echo "Body: {$this->message}\n";
    }
}

class SmsNotification extends Notification
{
    public function __construct(
        string $recipient,
        string $message,
        private string $phone
    ) {
        parent::__construct($recipient, $message);
    }

    // Override: extend parent behaviour with parent::
    public function send(): void
    {
        parent::send();   // run the parent version first
        echo "[SMS] Dispatched to {$this->phone}\n";
    }
}

$notifications = [
    new EmailNotification('alice', 'Hi!', 'Welcome'),
    new SmsNotification('bob',   'Hi!', '+44...'),
];

foreach ($notifications as $n) {
    $n->send();  // each calls its own version — polymorphism in action
}

class Payment
{
    // Children may not override this — the audit trail must always run
    final public function process(): void
    {
        $this->validate();
        $this->charge();
        $this->audit();   // always runs, cannot be bypassed
    }

    protected function validate(): void { /* ... */ }
    protected function charge(): void   { /* ... */ }
    private function   audit(): void    { /* log the transaction */ }
}

// Immutable value object — nobody should subclass this
final class Uuid
{
    public function __construct(public readonly string $value) {}
}

// ─── LSP VIOLATION ────────────────────────────────────────
class Rectangle
{
    public function __construct(
        protected int $width,
        protected int $height
    ) {}

    public function setWidth(int $w): void  { $this->width  = $w; }
    public function setHeight(int $h): void { $this->height = $h; }
    public function area(): int { return $this->width * $this->height; }
}

class Square extends Rectangle
{
    // Square must keep width === height, so it overrides BOTH setters
    public function setWidth(int $w): void
    {
        $this->width = $this->height = $w; // surprise side effect!
    }
    public function setHeight(int $h): void
    {
        $this->width = $this->height = $h; // surprise side effect!
    }
}

// Code that works for Rectangle breaks silently for Square:
function testArea(Rectangle $r): void
{
    $r->setWidth(5);
    $r->setHeight(4);
    assert($r->area() === 20);  // fails for Square! area = 16 (4*4)
}

// ─── CORRECT DESIGN ───────────────────────────────────────
// Make both Rectangle and Square independent — they just share a Shape interface
interface Shape
{
    public function area(): float;
}

class Rectangle implements Shape
{
    public function __construct(
        private readonly int $width,
        private readonly int $height
    ) {}
    public function area(): float { return $this->width * $this->height; }
}

class Square implements Shape
{
    public function __construct(private readonly int $side) {}
    public function area(): float { return $this->side ** 2; }
}

// ❌ Inheritance approach — Logger "is-a" FileWriter? No.
class Logger extends FileWriter
{
    public function log(string $msg): void
    {
        $this->write("[LOG] {$msg}");   // also inherits 50 file methods we don't want
    }
}

// ✅ Composition — Logger "has-a" writer (could be file, DB, cloud…)
interface Writer
{
    public function write(string $line): void;
}

class FileWriter implements Writer
{
    public function __construct(private string $path) {}

    public function write(string $line): void
    {
        file_put_contents($this->path, $line . PHP_EOL, FILE_APPEND);
    }
}

class Logger
{
    public function __construct(private Writer $writer) {}

    public function log(string $msg): void
    {
        $this->writer->write(date('[H:i:s] ') . $msg);
    }

    public function error(string $msg): void
    {
        $this->log("ERROR: {$msg}");
    }
}

// Swap the writer without touching Logger at all:
$logger = new Logger(new FileWriter('/var/log/app.log'));
$logger->log('Server started');

// In tests, swap for an in-memory writer — no file I/O needed:
class ArrayWriter implements Writer
{
    public array $lines = [];
    public function write(string $line): void { $this->lines[] = $line; }
}

$spy    = new ArrayWriter();
$logger = new Logger($spy);
$logger->log('test message');
assert(str_contains($spy->lines[0], 'test message')); // easy to test!