Pass Javascript variable to PHP via ajax

I am trying to pass a variable from my javascript code over to the server side PHP code. I know this must be done via an ajax call which i believe i have done correctly, however accessing the variable i pass from my ajax into my php is when i run into trouble as i am new to php. Here is my code i have thus far:

$(document).ready(function() {

            $(".clickable").click(function() {
                var userID = $(this).attr('id');
                //alert($(this).attr('id'));
                $.ajax({
                    type: "POST",
                    url: 'logtime.php',
                    data: "userID=" + userID,
                    success: function(data)
                    {
                        alert("success!");
                    }
                });
            });
        });

<?php //logtime.php
$uid = isset($_POST['userID']);
//rest of code that uses $uid
?>

I'm trying to pass my javascript variable "userID" to php ($userID), however i've gone wrong somewhere along the road. Thanks for the help!