I want to store php ajax success return value in a variable

php file

$('.routine-of-classes ul.nav li button').click(function(){
        $.ajax({
            url: 'ajax.php',
            type: 'POST',
            data: {
                className : $(this).attr("className")
            },
            success: function(data){
                $('.get-class-id').text(data);
            }
        });
    });

ajax file

if(isset($_POST['className'])){
    $getSubId = $_REQUEST['className'];
    echo $getSubId;
}

I want to store the return into a variable name like $class what should I need to do?