PHP if/elseif with isset condition

I have just started with php and need some help to understand.

These lines below do not work properly as I had imagined

if (isset ($_GET['submit']) && empty ($_SESSION['basket'])){
  $ _SESSION['basket'] = array($_GET['product'], $_GET['price'], $_GET['number']);

  print_r ($_SESSION['basket']);

}elseif (isset ($_GET['submit'])) {
   echo "already submitted";
}

The first time you press submit, the basket is empty, which means that the first code is running. But the second time I press submit, it should not be empty anymore? But still, it does not run the other code.

What have I missed? Please give me some tips