Drupal Commerce add line item

Function for programmatically add a product as line item by using product id

function product_cart_add($product_id, $quantity, $uid = NULL) {
  $line_item = NULL;
  if ($uid == NULL) {
    global $user;
    $uid = $user->uid;
  }
  if ($product = commerce_product_load($product_id)) {
    $line_item = commerce_product_line_item_new($product, $quantity);
    $line_item = commerce_cart_product_add($uid, $line_item);
  }
  return $line_item;
}