public static function set(string $key, $value) : void
    {
        $_SESSION[Notification::SESSION_KEY][$key] = $value;
    }
public static function set(string $key, $value) : void
    {
        $_SESSION[Notification::SESSION_KEY][$key] = $value;
    }
public static function set ( string $ key , $ value ) : void { $ _SESSION [ Notification :: SESSION_KEY ] [ $ key ] = $ value ; }
Adds specific variable to the Notifications array.

Notification::set('success', 'Data has been saved with success!');
Notification::set('errors', 'Data not saved!');

@param string $key   Variable name
@param mixed  $value Variable value
Adds specific variable to the Notifications array .