Adds a 'Mark post unread' button to each post in viewtopic.php, adds a 'Mark forums read' link to the 'Unread posts' search results and optionally changes the 'Unread posts' search link text to either 'No unread posts' or 'Unread posts in X topics'.
More details: https://www.phpbb.com/customise/db/extension/mark_post_unread/.