Abstract class, interface, and trait example.

Autoloader.php

<?php
class Autoloader
{
    /**
     * @param string $className
     */
    public function autoload($className)
    {
        require str_replace('\\', '/', $className) . '.php';
    }

    public function register()
    {
        spl_autoload_register([$this, 'autoload']);
    }
}

DownloadableProduct.php

<?php
namespace Company;

use JsonSerializable;

class DownloadableProduct extends Product implements JsonSerializable
{
    use ProvidesJsonEncoding;

    /** @type string */
    protected $file;

    /**
     * @param string $name
     * @param Money  $price
     * @param string $file
     */
    public function __construct($name, Money $price, $file)
    {
        parent::__construct($name, $price);
        $this->file = $file;
    }
}

Money.php

<?php
namespace Company;

use JsonSerializable;

class Money implements JsonSerializable
{
    use ProvidesJsonEncoding;

    /** @type integer */
    protected $amount;

    /** @type string */
    protected $currency;

    /**
     * @param integer $amount
     * @param string $currency
     */
    public function __construct($amount, $currency)
    {
        $this->amount = $amount;
        $this->currency = $currency;
    }
}

Product.php

<?php
namespace Company;

abstract class Product
{
    /** @type string */
    protected $name;

    /** @type Money */
    protected $price;

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

ProvidesJsonEncoding.php

<?php
namespace Company;

trait ProvidesJsonEncoding
{
    /**
     * @return array
     */
    public function jsonSerialize()
    {
        $json = get_object_vars($this);
        foreach ($json as $key => $property) {
            if ($property instanceof CastsToJson) {
                $json[$key] = $property->jsonSerialize();
            }
        }
        return $json;
    }
}