How To Read JSON from external PHP in JQuery on success

Lets say you want to check if your query successfuly executed.
 The javascript JQuery whould be like this :
$.ajax({
        type: "POST", url: "./update.php",
        data : { 'id' : $("#id").val() },
        success: function(data)
        {
                var obj = jQuery.parseJSON(data);
                 alert(obj[0].result);
        }
});

And the PHP script of update.php like this :
include "./includes/database.php";
if (mysql_query("select id from table where id = '".$_POST['id']."'"))
{
        $return[] = array('result' => 'id exists');
}
else
{
        $return[] = array('result' => 'id not found');
}
echo json_encode($return);