如何使用苗条的$ _FILES []

I'm trying to take data from a form by post with slim, but the documentation does not show how to use the $_FILES[], I can take the name of the file but does not move to the folder.

 $app->post('/publicar', function () use ($app, $db){

  $img = $_FILES['img']['name'];
  $ruta = $_FILES['img']['tmp_name'];
  $destino = "images/".$img;
  move_uploaded_file($ruta, $destino);
});