Difference between filter_var and filter_input on input data validation

Question

What's the difference between using one of these two functions when validating user input? Strictly regarding the function calls, there's no requirement to change the $_POST array for instance.

$result = filter_var($_POST['user_input'], FILTER_VALIDATE_INT);

vs

$result = filter_input(INPUT_POST, 'user_input', FILTER_VALIDATE_INT);

Or there's no difference between the two calls above, not even performance wise, but rather just a matter of preference?

PS: I know there's a similar question on SO - Differences between filter_var and filter_input - but that just states how the 2 methods should be called, not what's the actual difference.

Answer 1

If the request body does not contain the parameter user_input at all, $_POST['user_input'] will trigger a notice, filter_input(INPUT_POST, 'user_input', ..) won't.