请大家帮忙看看有关问题

请大家帮忙看看问题
程序是要输入两个json格式的txt文件，然后通过匹配两个文件的相同的index，然后输出一个新的json格式的txt文件。 比如：
这是输入的两个文件的格式：
1. Product
{
"product_name": String // A unique id for the product
"manufacturer": String
"family": String // optional grouping of products
"model": String
"announced-date": String // ISO-8601 formatted date string, e.g. 2011-04-28T19:00:00.000-05:00
}
2. Listing
{
"title": String // description of product for sale
"manufacturer": String // who manufactures the product for sale
"currency": String // currency code, e.g. USD, CAD, GBP, etc.
"price": String // price, e.g. 19.99, 100.00
}

输出的文件格式为：
{
"product_name": String
"listings": Array[Listing]
}

先上代码：
BaseItem.php

abstract class BaseItem

{

    /**

     * @param array $fields

     */

    public function __construct(array $fields)

    {

        $this->buildFromArray($fields);

    }



    /**

     * @param array $fields

     */

    abstract public function buildFromArray(array $fields);



    /**

     * @return array

     */

    abstract public function convertToArray();

}

?>

Listing.php

include_once 'BaseItem.php';

class Listing extends BaseItem

{

    protected $title;

    protected $manufacturer;

    protected $currency;

    protected $price;



    /**

     * @inheritDoc

     */

    public function convertToArray()

    {

        return array(

            'title' => $this->title,

            'manufacturer' => $this->manufacturer,

            'currency' => $this->currency,

            'price' => $this->price,

        );

    }



    /**

     * @inheritDoc

     */

    public function buildFromArray(array $fields)

    {

        $this->title = $fields['title'];

        $this->manufacturer = $fields['manufacturer'];