function messages_format_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string' ) {
	$total_items = (int) $total_items;
	$text        = '';
	$link        = trailingslashit( bp_loggedin_user_domain() . bp_get_messages_slug() . '/inbox' );
	$title       = __( 'Inbox', 'buddypress' );
	$amount      = 'single';

	if ( 'new_message' === $action ) {
		if ( $total_items > 1 ) {
			$amount = 'multiple';
			$text   = sprintf( __( 'You have %d new messages', 'buddypress' ), $total_items );

		} else {
			// Get message thread ID.
			$message   = new BP_Messages_Message( $item_id );
			$thread_id = $message->thread_id;
			$link      = ( ! empty( $thread_id ) )
				? bp_get_message_thread_view_link( $thread_id )
				: false;

			if ( ! empty( $secondary_item_id ) ) {
				$text = sprintf( __( '%s sent you a new private message', 'buddypress' ), bp_core_get_user_displayname( $secondary_item_id ) );
			} else {
				$text = sprintf( _n( 'You have %s new private message', 'You have %s new private messages', $total_items, 'buddypress' ), bp_core_number_format( $total_items ) );
			}
		}

		if ( 'string' === $format ) {
			if ( ! empty( $link ) ) {
				$return = '<a href="' . esc_url( $link ) . '">' . esc_html( $text ) . '</a>';
			} else {
				$return = esc_html( $text );
			}

			/**
			 * Filters the new message notification text before the notification is created.
			 *
			 * This is a dynamic filter. Possible filter names are:
			 *   - 'bp_messages_multiple_new_message_notification'.
			 *   - 'bp_messages_single_new_message_notification'.
			 *
			 * @param string $return            Notification text.
			 * @param int    $total_items       Number of messages referred to by the notification.
			 * @param string $text              The raw notification test (ie, not wrapped in a link).
			 * @param int    $item_id           ID of the associated item.
			 * @param int    $secondary_item_id ID of the secondary associated item.
			 */
			$return = apply_filters( 'bp_messages_' . $amount . '_new_message_notification', $return, (int) $total_items, $text, $link, $item_id, $secondary_item_id );
		} else {
			/** This filter is documented in bp-messages/bp-messages-notifications.php */
			$return = apply_filters( 'bp_messages_' . $amount . '_new_message_notification', array(
				'text' => $text,
				'link' => $link
			), $link, (int) $total_items, $text, $link, $item_id, $secondary_item_id );
		}

	// Custom notification action for the Messages component
	} else {
		if ( 'string' === $format ) {
			$return = $text;
		} else {
			$return = array(
				'text' => $text,
				'link' => $link
			);
		}

		/**
		 * Backcompat for plugins that used to filter bp_messages_single_new_message_notification
		 * for their custom actions. These plugins should now use 'bp_messages_' . $action . '_notification'
		 */
		if ( has_filter( 'bp_messages_single_new_message_notification' ) ) {
			if ( 'string' === $format ) {
				/** This filter is documented in bp-messages/bp-messages-notifications.php */
				$return = apply_filters( 'bp_messages_single_new_message_notification', $return, (int) $total_items, $text, $link, $item_id, $secondary_item_id );

			// Notice that there are seven parameters instead of six? Ugh...
			} else {
				/** This filter is documented in bp-messages/bp-messages-notifications.php */
				$return = apply_filters( 'bp_messages_single_new_message_notification', $return, $link, (int) $total_items, $text, $link, $item_id, $secondary_item_id );
			}
		}

		/**
		 * Filters the custom action notification before the notification is created.
		 *
		 * This is a dynamic filter based on the message notification action.
		 *
		 * @since 2.6.0
		 *
		 * @param array  $value             An associative array containing the text and the link of the notification
		 * @param int    $item_id           ID of the associated item.
		 * @param int    $secondary_item_id ID of the secondary associated item.
		 * @param int    $total_items       Number of messages referred to by the notification.
		 * @param string $format            Return value format. 'string' for BuddyBar-compatible
		 *                                  notifications; 'array' for WP Toolbar. Default: 'string'.
		 */
		$return = apply_filters( "bp_messages_{$action}_notification", $return, $item_id, $secondary_item_id, $total_items, $format );
	}

	/**
	 * Fires right before returning the formatted message notifications.
	 *
	 * @since 1.0.0
	 *
	 * @param string $action            The type of message notification.
	 * @param int    $item_id           The primary item ID.
	 * @param int    $secondary_item_id The secondary item ID.
	 * @param int    $total_items       Total amount of items to format.
	 */
	do_action( 'messages_format_notifications', $action, $item_id, $secondary_item_id, $total_items );

	return $return;
}