PHP Class Database interaction (using PDO)

Hey guys, so I'm completely new to Object Oriented PHP -- I've read some tutorials, but I can't find anything that really goes into working with a database with PHP classes.

I'm trying to make something simple -- a quick news post class. You get the post from the database, etc. However, I'm getting an error whenever I try to interact with the database.

I read that PDO is the way to go with OO PHP; to that end, I've developed a database class, as detailed in this post: Use of PDO in classes

class Database
{
    public $db;   // handle of the db connexion
    private static $dsn  = "mysql:host=localhost;dbname=test";
    private static $user = "admin";
    private static $pass = "root";
    private static $instance;

    public function __construct () 
    {
        $this->db = new PDO(self::$dsn,self::$user,self::$pass);
    }

    public static function getInstance()
    {
        if(!isset(self::$instance))
        {
            $object= __CLASS__;
            self::$instance=new $object;
        }
        return self::$instance;
    }

    // others global functions
}

I then attempt to use it in my PHP class, in order to retrieve data on a news post:

<?php
require_once "database.php";

class news extends Database
{
    private $title;
    private $author;
    private $date;
    private $content;
    private $category;


    function __construct($id)
    {
        $db = Database::getInstance();
        $query = $this->db->prepare("SELECT title, author, date, content, category FROM news WHERE id = :id LIMIT 1");
        $query->bindParam(":id", $this->id, PDO::PARAM_INT);
        if ($query->execute())
        {
            $result = $query->fetch(PDO::FETCH_OBJ);

            $this->set_title($result->title);
            $this->set_author($result->author);
            $this->set_date($result->date);
            $this->set_content($result->content);
            $this->set_category($result->category);
        }
    }
<...>
?>

Every time I try to run this script though, I get the following error:

Fatal error: Call to a member function prepare() on a non-object in /news.class.php on line 16

Any ideas?