// We have 'tasks' table with PK 'sno'.

$e["on_update"] = array("log_data", null, true);
$e["on_insert"] = array("log_data", null, true);
$e["on_delete"] = array("log_data", null, true);
$grid->set_events($e);

function log_data($data)
{
    $rs = mysql_fetch_assoc(mysql_query("SELECT * FROM tasks WHERE sno = {$data[sno]}"));
    $data["old"] = $rs;

    if ($_POST["oper"] == "add") $oper = "Added";
    else if ($_POST["oper"] == "edit") $oper = "Updated";
    else if ($_POST["oper"] == "del") $oper = "Deleted";

    // here $data = new data, $data["old"] = old data and $oper is operation.

    $change_str = "";
    if (!empty($data["old"]))
    {
        // changelog track
        $changes = array();
        foreach ($data["params"] as $k=>$v)
            foreach ($data["old"] as $ok => $ov)
                if ($k == $ok && $v != $ov)
                {
                    $ov = empty($ov) ? "(blank)" : $ov;
                    $changes[] = "$k: $ov -> $v";
                }
        $change_str = implode(", ",$changes);
    }

    // execute the logging query
}

// Note that with on_delete event, you only get $data["sno"] and no new data (due to obvious reason)