bp_activity_format_notifications( string $action, int $item_id, int $secondary_item_id, int $total_items, string $format = 'string', int $id )

function bp_activity_format_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string', $id = 0 ) {
	$action_filter = $action;
	$return        = false;
	$activity_id   = $item_id;
	$user_id       = $secondary_item_id;
	$user_fullname = bp_core_get_user_displayname( $user_id );

	switch ( $action ) {
		case 'new_at_mention':
			$action_filter = 'at_mentions';
			$link          = bp_activity_get_permalink($item_id);
			$title         = sprintf( __( '@%s Mentions', 'buddyboss' ), bp_get_loggedin_user_username() );
			$amount        = 'single';

			if ( (int) $total_items > 1 ) {
				$text   = sprintf( __( 'You have %1$d new mentions', 'buddyboss' ), (int) $total_items );
				$amount = 'multiple';
			} else {
				$text = sprintf( __( '%1$s mentioned you', 'buddyboss' ), $user_fullname );
			}
		break;

		case 'update_reply':
			$link   = bp_get_notifications_permalink();
			$title  = __( 'New Activity reply', 'buddyboss' );
			$amount = 'single';

			if ( (int) $total_items > 1 ) {
				$link   = add_query_arg( 'type', $action, $link );
				$text   = sprintf( __( 'You have %1$d new replies', 'buddyboss' ), (int) $total_items );
				$amount = 'multiple';
			} else {
				$link = add_query_arg( 'rid', (int) $id, bp_activity_get_permalink( $activity_id ) );
				$text = sprintf( __( '%1$s commented on one of your updates', 'buddyboss' ), $user_fullname );
			}
		break;

		case 'comment_reply':
			$link   = bp_get_notifications_permalink();
			$title  = __( 'New Activity comment reply', 'buddyboss' );
			$amount = 'single';

			if ( (int) $total_items > 1 ) {
				$link   = add_query_arg( 'type', $action, $link );
				$text   = sprintf( __( 'You have %1$d new comment replies', 'buddyboss' ), (int) $total_items );
				$amount = 'multiple';
			} else {
				$link = add_query_arg( 'crid', (int) $id, bp_activity_get_permalink( $activity_id ) );
				$text = sprintf( __( '%1$s replied to one of your activity comments', 'buddyboss' ), $user_fullname );
			}
		break;
	}

	if ( 'string' == $format ) {

		/**
		 * Filters the activity notification for the string format.
		 *
		 * This is a variable filter that is dependent on how many items
		 * need notified about. The two possible hooks are bp_activity_single_at_mentions_notification
		 * or bp_activity_multiple_at_mentions_notification.
		 *
		 * @since BuddyPress 1.5.0
		 * @since BuddyPress 2.6.0 use the $action_filter as a new dynamic portion of the filter name.
		 *
		 * @param string $string          HTML anchor tag for the interaction.
		 * @param string $link            The permalink for the interaction.
		 * @param int    $total_items     How many items being notified about.
		 * @param int    $activity_id     ID of the activity item being formatted.
		 * @param int    $user_id         ID of the user who inited the interaction.
		 */
		$return = apply_filters( 'bp_activity_' . $amount . '_' . $action_filter . '_notification', '<a href="' . esc_url( $link ) . '">' . esc_html( $text ) . '</a>', $link, (int) $total_items, $activity_id, $user_id );
	} else {

		/**
		 * Filters the activity notification for any non-string format.
		 *
		 * This is a variable filter that is dependent on how many items need notified about.
		 * The two possible hooks are bp_activity_single_at_mentions_notification
		 * or bp_activity_multiple_at_mentions_notification.
		 *
		 * @since BuddyPress 1.5.0
		 * @since BuddyPress 2.6.0 use the $action_filter as a new dynamic portion of the filter name.
		 *
		 * @param array  $array           Array holding the content and permalink for the interaction notification.
		 * @param string $link            The permalink for the interaction.
		 * @param int    $total_items     How many items being notified about.
		 * @param int    $activity_id     ID of the activity item being formatted.
		 * @param int    $user_id         ID of the user who inited the interaction.
		 */
		$return = apply_filters( 'bp_activity_' . $amount . '_' . $action_filter . '_notification', array(
			'text' => $text,
			'link' => $link
		), $link, (int) $total_items, $activity_id, $user_id );
	}

	/**
	 * Fires right before returning the formatted activity notifications.
	 *
	 * @since BuddyPress 1.2.0
	 *
	 * @param string $action            The type of activity item.
	 * @param int    $item_id           The activity ID.
	 * @param int    $secondary_item_id The user ID who inited the interaction.
	 * @param int    $total_items       Total amount of items to format.
	 */
	do_action( 'activity_format_notifications', $action, $item_id, $secondary_item_id, $total_items );

	return $return;
}