How do we materialize " [url]?parameter= " in php?

Question

I want to make a condition so a if a user pass ?parameter=
With just an equal and no value after , then I do something

Answer 1

either using if(isset($_GET['parameter'])) or if(!empty($_GET['parameter']))
not sure which one

Answer 2

if ($_GET['parameter'] === '')

or

if (empty($_GET['parameter']))