$db = new PDO('sqlite:base.sqlite');

$st = $db->prepare("SELECT id FROM ostatki WHERE code=:art");
$st->execute(array('art' => $art));
$results = $st->fetchAll();

$insert_db = $db->prepare("INSERT INTO 'ostatki' ('code', 'name', 'unit', 'quantity', 'brand') VALUES (:art, :name, :unit, :quantity, :brand)");
$insert_db->execute(array(
    'art' => $art,
    'name' => $name,
    'unit' => $unit,
    'quantity' => $quantity,
    'brand' => $brand
    ));

$update_db = $db->prepare("UPDATE ostatki SET name=:name, quantity=:quantity, unit=:unit WHERE id = :id");
$update_db->execute(array(
    'id' => $id,
    'name' => $name,
    'unit' => $unit,
    'quantity' => $quantity,
));