example.php

<?php

class ShoppingCart {
  protected $lineItems = [];

  public function add($lineItem)
  {
    $this->lineItems[] = $lineItem;
  }

  public function getTotal()
  {
    $total = 0;

    foreach($this->lineItems as $lineItem) {
      $total += $lineItem->getTotal();
    }

    return $total;
  }
}

class LineItem {
  public $product, $quantity;

  public function __construct(Product $product, int $quantity)
  {
    $this->product = $product;
    $this->quantity = $quantity;
  }

  public function getTotal()
  {
    return $this->quantity * $this->product->price;
  }
}

class Product {
  public $name, $price;

  public function __construct($name, $price)
  {
    $this->name = $name;
    $this->price = $price;
  }
}

$album = new Product('Bright Soundtrack', 10.99);
$song = new Product('The Break Up by MGK', 0.99);
$show = new Product('Big Bang Theory Season 1', 34.99);

$cart = new ShoppingCart();
$cart->add(new LineItem($album, 1));
$cart->add(new LineItem($song, 1));
$cart->add(new LineItem($show, 1));
echo $cart->getTotal();
echo '<br>';

// Example 2
class Track extends Product {
  protected $bytes;

  public static function find($id, $pdo)
  {
    $statement = $pdo->prepare('
      select *
      from tracks
      where TrackId = ?
    ');
    $statement->bindParam(1, $id);
    $statement->execute();
    $track = $statement->fetch(PDO::FETCH_OBJ);
    return new Track($track);
  }

  public function __construct($track)
  {
    $this->name = $track->Name;
    $this->price = $track->UnitPrice;
    $this->bytes = $track->Bytes;
  }

  public function getSize($size)
  {
    switch ($size) {
      case 'B':
        return $this->bytes;
      case 'KB':
        return $this->bytes * pow(10, -3);
      case 'MB':
        return $this->bytes * pow(10, -6);
    }
  }
}
$pdo = new PDO('sqlite:chinook.db');
$track = Track::find(2166, $pdo);
echo $track->getSize('MB');
echo '<br>';
$cart->add(new LineItem($track, 1));
echo $cart->getTotal();
echo '<br>';