How do I programatically empty trash?

I need to create an “Empty Trash” button inside my plugin. How would I do it using PHP code?

You can use wp_delete_post.

To get all posts with the “trash” status:

$trash = get_posts('post_status=trash&numberposts=-1');

Then:

foreach($trash as $post)
  wp_delete_post($post->ID, $bypass_trash = true);