HTML a href link:

<a href=# id=clickme>Update Session</a>

jQuery codes:

You can send data with line of data: { varname: 'here we go'}

<script src=http://code.jquery.com/jquery-1.10.1.min.js></script>

<script>

$(#clickme).click(function(e) {

    e.preventDefault();

    $.ajax({

        type:'POST',

        url:'update.php',

        data: { varname: 'here we go'},

        success:function(response){

           alert(response);

        }

    });

});

</script>

update.php file:

You can get sent var with $_POST[varname]

<?php

   $_SESSION[varname] = $_POST[varname];

   echo $_SESSION[varname];

?>