Dependency Injection in PHP

gistfile1.php

    <?php
     
    /**
    * Product class
    */
    class Product {
     
    /**
    * database layer to be used
    * @access protected
    * @var DB
    */
    protected $db;
     
    /**
    * Creates Product
    * @param string $db for example
    * @param array $fields product fields
    */
    public function __construct($db,$fields)
    {
    $this->db = $db;
    $this->fields = $fields;
    }
     
    /**
    * retreive products by id
    * @param int $id product id
    * @return mixed
    */
    public function getProductById($id)
    {
    return $this->db->findById($id);
    }
    /**
    * find product by executing a query
    * @param array $query array of query options
    * @return mixed
    */
    public function getProductByQuery($query)
    {
    return $this->db->execQuery($query);
    }
    //other operations....
    }
     
    $db = new DB($dbParams);// creating DB object
    $product = new Product($db,$fields);//injecting DB object through constructor