class odb
{
    static $instance = null;
    public static function getInstance()
    {
        if (null === $instance) {
            $instance = new static();
        }
        return $instance;
    }
    final private function __clone(){}        // prevent cloning
    final private function __wakeup(){}       // prevent unserializing

    private $PDOHost =        'localhost';
    private $PDOUser =        'username';
    private $PDOPass =        'password';
    private $PDODB   =        'database';
    private $PDO     =        null;

    protected function __construct()
    {
        $this->PDO = new PDO('mysql:host=' . $this->PDOHost . ';' .'dbname=' . $this->PDODB, $this->PDOUser, $this->PDOPass);
    }

}

try{

    $stmt = odb::getInstance()->prepare("SELECT COUNT(*) FROM `users` WHERE `username` = :user AND `password` = :pass");
    $stmt->execute(array(":user" => "tesusername", ":pass" => "testpassword"));

    print_r($stmt->fetchAll());


}catch(Exception $e){
    die("oops");
}