bbp_notify_forum_subscribers( int $topic_id, int $forum_id, mixed $anonymous_data = false, int $topic_author )

function bbp_notify_forum_subscribers( $topic_id = 0, $forum_id = 0, $anonymous_data = false, $topic_author = 0 ) {

	// Bail if subscriptions are turned off
	if ( !bbp_is_subscriptions_active() ) {
		return false;
	}

	/** Validation ************************************************************/

	$topic_id = bbp_get_topic_id( $topic_id );
	$forum_id = bbp_get_forum_id( $forum_id );

	/**
	 * Necessary for backwards compatibility
	 *
	 * @see https://bbpress.trac.wordpress.org/ticket/2620
	 */
	$user_id  = 0;

	/** Topic *****************************************************************/

	// Bail if topic is not published
	if ( ! bbp_is_topic_published( $topic_id ) ) {
		return false;
	}

	// Poster name
	$topic_author_name = bbp_get_topic_author_display_name( $topic_id );

	/** Mail ******************************************************************/

	// Remove filters from reply content and topic title to prevent content
	// from being encoded with HTML entities, wrapped in paragraph tags, etc...
	remove_all_filters( 'bbp_get_topic_content' );
	remove_all_filters( 'bbp_get_topic_title'   );

	// Strip tags from text and setup mail data
	$topic_title   = strip_tags( bbp_get_topic_title( $topic_id ) );
	$topic_content = strip_tags( bbp_get_topic_content( $topic_id ) );
	$topic_url     = get_permalink( $topic_id );
	$forum_title   = wp_strip_all_tags( get_post_field( 'post_title', $forum_id ) );
	$forum_url     = esc_url( bbp_get_forum_permalink( $forum_id ) );

	$args = array(
		'tokens' => array(
			'forum.title'        => $forum_title,
			'forum.url'          => $forum_url,
			'discussion.id'		 => $topic_id,
			'discussion.title'   => $topic_title,
			'discussion.url'     => $topic_url,
			'discussion.content' => $topic_content,
			'poster.name'        => $topic_author_name,
		),
	);

	// Get topic subscribers and bail if empty
	$user_ids = bbp_get_forum_subscribers( $forum_id, true );

	// Dedicated filter to manipulate user ID's to send emails to
	$user_ids = apply_filters( 'bbp_forum_subscription_user_ids', $user_ids );
	if ( empty( $user_ids ) ) {
		return false;
	}

	/** Send it ***************************************************************/

	do_action( 'bbp_pre_notify_forum_subscribers', $topic_id, $forum_id, $user_ids );

	// Loop through users
	foreach ( (array) $user_ids as $user_id ) {

		// Don't send notifications to the person who made the post
		if ( !empty( $topic_author ) && (int) $user_id === (int) $topic_author ) {
			continue;
		}

		// Send notification email.
		bp_send_email( 'bbp-new-forum-topic', (int) $user_id, $args );
	}

	do_action( 'bbp_post_notify_forum_subscribers', $topic_id, $forum_id, $user_ids );

	return true;
}