Snippet to create a link with a class programmatically.
use Drupal\Core\Link;
use Drupal\Core\Url;
$link_options = array(
'attributes' => array(
'class' => array(
'button',
'bg-green'
),
),
);
$url = Url::fromUri('internal:/node/'.$nid);
$url->setOptions($link_options);
$link = Link::fromTextAndUrl('Click here', $url)->toString();