How can __clone be used to customize what happens when an object is duplicated using PHP's clone keyword, particularly for handling deep copies of nested objects?

By default, PHP's clone keyword creates a shallow copy of an object, meaning any properties that are themselves objects will still reference the exact same underlying object instance in both the original and the clone, and implementing the __clone magic method lets you customize this behavior, such as explicitly cloning nested objects as well, ensuring the resulting clone is genuinely independent of the original rather than unintentionally sharing internal object references.

class Order {
    public Customer $customer;
    public function __clone() {
        $this->customer = clone $this->customer;
    }
}
$original = new Order();
$copy = clone $original;