Answers for "whats are set"

what is set

SET: Can only store unique values, 
     And does not maintain order
- HashSet can have null, order is not guaranteed
- LinkedHashSet can have null and keeps the order 
- TreeSet sorts the order and don't accept null

is set

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
        if (isset($_POST['...'])) {

        } else {
            echo "...";
        }
    }