What does $_GET['key'] return if the key is not set?
$_GET is just an ordinary array, so it behaves exactly the same as any other array.
This means, it will return NULL to the variable and raise the "undefined index" notice when you call a non-existing index.
The only thing you need to be aware of with $_GET is that is contains unsafe (user-modifiable) data
var_dump($_GET['nonexistent']); // outputs NULL