session value changes after changing of associated variable

I have a code for cart using session. In shopping cart page I assigned the variable to session for fetch the cart and show it but when I change some values of variable, my session changed automatically and in next refresh of page, my session values updated.

I'm using laravel 5.

And this is my code:

  $oldCart = Session::get('cart');
  foreach($oldCart as  $cartRow):
    foreach($cartRow as $crKey => $row):
      $oldCart->totalPrice -= 2000;
      $oldCart->items[$crKey]['price'] -= 1000;
      endforeach;
  endforeach;

In code above, The value of 'total price' in session decreases 1000 in each refresh of page but I don't want to change any value of session.