gistfile1.php

<?php

class Order
{
    /**
     * @var OrderLine[]
     */
    private $orderLines = [];
    
    /**
     * @param string $name
     * @param float  $quantity
     */
    public function addOrderLine($name, $quantity)
    {
        $this->orderLines = new OrderLine($name, $quantity);
    }
}

class OrderLine
{
    /**
     * @var string
     */
    private $name;

    /**
     * @var float
     */
    private $quantity;

    /**
     * @param string $name
     * @param float  $quantity
     */
    public function __construct($name, $quantity)
    {
        $this->name = $name;
        $this->quantity = $quantity;
    }
}

// Default Order usage as it was intended to use
$sampleOrder = new Order();
$sampleOrder->addOrderLine("Product A", 1.0);
$sampleOrder->addOrderLine("Product B", 1.0);

// Now we have a special requirement: our order lines now must include a source
class CustomOrderLine extends OrderLine
{
    /**
     * @var string
     */
    private $source;

    /**
     * @param string $name
     * @param float  $quantity
     * @param string $source
     */
    public function __construct($name, $quantity, $source)
    {
        parent::__construct($name, $quantity);

        $this->source = $source;
    }
}

// But we cannot change the signature of `addOrderLine`, as PHP will throw a
// strict warning about a method signature mismatch
class CustomOrder extends Order
{
    // !!! Notice Declaration of (...) should be compatible with that of (...)
    public function addOrderLine($name, $quantity, $source)
    {
        ...
    }
}

// So, can't we extend `Order`? Do we have to implement all methods from scratch?