bbp_update_forum_last_reply_id( int $forum_id, int $reply_id )

function bbp_update_forum_last_reply_id( $forum_id = 0, $reply_id = 0 ) {
	$forum_id = bbp_get_forum_id( $forum_id );

	// Define local variable(s)
	$children_last_reply = 0;

	// Do some calculation if not manually set
	if ( empty( $reply_id ) ) {

		// Loop through children and get the most recent reply id
		$children = bbp_forum_query_subforum_ids( $forum_id );
		if ( !empty( $children ) ) {
			foreach ( (array) $children as $child ) {
				$children_last_reply = bbp_update_forum_last_reply_id( $child ); // Recursive
			}
		}

		// If this forum has topics...
		$topic_ids = bbp_forum_query_topic_ids( $forum_id );
		if ( !empty( $topic_ids ) ) {

			// ...get the most recent reply from those topics...
			$reply_id = bbp_forum_query_last_reply_id( $forum_id, $topic_ids );

			// ...and compare it to the most recent topic id...
			$reply_id = ( $reply_id > max( $topic_ids ) ) ? $reply_id : max( $topic_ids );
		}
	}

	// Cast as integer in case of empty or string
	$reply_id            = (int) $reply_id;
	$children_last_reply = (int) $children_last_reply;

	// If child forums have higher ID, check for newer reply id
	if ( !empty( $children ) && ( $children_last_reply > $reply_id ) )
		$reply_id = $children_last_reply;

	// Update the last public reply ID
	if ( bbp_is_reply_published( $reply_id ) )
		update_post_meta( $forum_id, '_bbp_last_reply_id', $reply_id );

	return (int) apply_filters( 'bbp_update_forum_last_reply_id', $reply_id, $forum_id );
}