How do I fix undefined property in .php file

I am new to PHP and I’ve encountered an error an error I’m not able to fix. I have an program that giving out an error in VSCode written “undefined property ‘$router'” How do i fix this

$_module OR $_module = CI::$APP->router->fetch_module();
        list($path, $file) = Modules::find($file, $_module, 'config/');