function friends_format_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string' ) {

	switch ( $action ) {
		case 'friendship_accepted':
			$link = trailingslashit( bp_loggedin_user_domain() . bp_get_friends_slug() . '/my-friends' );

			// $action and $amount are used to generate dynamic filter names.
			$action = 'accepted';

			// Set up the string and the filter.
			if ( (int) $total_items > 1 ) {
				$text = sprintf( __( '%d friends accepted your friendship requests', 'buddypress' ), (int) $total_items );
				$amount = 'multiple';
			} else {
				$text = sprintf( __( '%s accepted your friendship request', 'buddypress' ),  bp_core_get_user_displayname( $item_id ) );
				$amount = 'single';
			}

			break;

		case 'friendship_request':
			$link = bp_loggedin_user_domain() . bp_get_friends_slug() . '/requests/?new';

			$action = 'request';

			// Set up the string and the filter.
			if ( (int) $total_items > 1 ) {
				$text = sprintf( __( 'You have %d pending friendship requests', 'buddypress' ), (int) $total_items );
				$amount = 'multiple';
			} else {
				$text = sprintf( __( 'You have a friendship request from %s', 'buddypress' ),  bp_core_get_user_displayname( $item_id ) );
				$amount = 'single';
			}

			break;
	}

	// Return either an HTML link or an array, depending on the requested format.
	if ( 'string' == $format ) {

		/**
		 * Filters the format of friendship notifications based on type and amount * of notifications pending.
		 *
		 * This is a variable filter that has four possible versions.
		 * The four possible versions are:
		 *   - bp_friends_single_friendship_accepted_notification
		 *   - bp_friends_multiple_friendship_accepted_notification
		 *   - bp_friends_single_friendship_request_notification
		 *   - bp_friends_multiple_friendship_request_notification
		 *
		 * @since 1.0.0
		 *
		 * @param string|array $value       Depending on format, an HTML link to new requests profile
		 *                                  tab or array with link and text.
		 * @param int          $total_items The total number of messaging-related notifications
		 *                                  waiting for the user.
		 * @param int          $item_id     The primary item ID.
		 */
		$return = apply_filters( 'bp_friends_' . $amount . '_friendship_' . $action . '_notification', '<a href="' . esc_url( $link ) . '">' . esc_html( $text ) . '</a>', (int) $total_items, $item_id );
	} else {
		/** This filter is documented in bp-friends/bp-friends-notifications.php */
		$return = apply_filters( 'bp_friends_' . $amount . '_friendship_' . $action . '_notification', array(
			'link' => $link,
			'text' => $text
		), (int) $total_items, $item_id );
	}

	/**
	 * Fires at the end of the bp-friends notification format callback.
	 *
	 * @since 1.0.0
	 *
	 * @param string       $action            The kind of notification being rendered.
	 * @param int          $item_id           The primary item ID.
	 * @param int          $secondary_item_id The secondary item ID.
	 * @param int          $total_items       The total number of messaging-related notifications
	 *                                        waiting for the user.
	 * @param array|string $return            Notification text string or array of link and text.
	 */
	do_action( 'friends_format_notifications', $action, $item_id, $secondary_item_id, $total_items, $return );

	return $return;
}