Why check $_GET twice?

Question

I'm learning PHP and I hoping you can explain why the author of a shopping cart tutorial has done this in a call to add a product to a cart.

if(isset($_GET['action']) && $_GET['action']=="add")

The site checks that $_GET[action] is set and that $_GET[action] is equal to 'add' before processing but isn't this functionally equivalent to just:

if($_GET['action']=="add")

Because if the value is 'add' it has to be set, and if it's not set it can't possibly be 'add' right? Or am I missing something?

Answer 1

No, thats not the same Let me explaint 1. isset check either key is set in array 2. Double equals check the simple comparison