It doesn’t really matter. If you simply must have it inside a class, I would use a constant and a static method.
// in the main plugin file
define( 'MYPLUGIN_FILE', __FILE__ );
// include another file with this class in
class MyPlugin {
public static function init() {
register_activation_hook( MYPLUGIN_FILE, array( 'MyPlugin', 'install' ));
}
public static function install() {
echo "little lamb, who made thee? is it wordpress or is it me";
}
}
// call the static method
MyPlugin::init();