Not Storing UserID

$data = array();
$data["ModBy"] = $_SESSION["UserID"];
$data["LID"] = $values["LID"];
$data["Disposition"] = $values["Disposition"];
$data["CallBackDate"] = $values["CallBackDate"];
$data["CallBackTime"] = $values["CallBackTime"];
$data["Comments"] = $values["Comments"];
$data["ModDate"] = $values["ModDate"];
DB::Insert("Disposition", $data )*

Above code is in Event > After record updated , on Edit Page.
Everything is fine but $data["ModBy"] = $SESSION["UserID"]; is not stroring UserID in database.
Could you please, right the correct one. Thanks