Include surplus of one campaign in total of another campaign

function.php

<?php
add_filter(
    'charitable_campaign_donated_amount',
    function( $amount, $campaign, $sanitize ) {
        $new_campaign = 14;
        $old_campaign = 135;
        $old_goal     = charitable_get_campaign( $old_campaign )->get_goal();

        if ( $new_campaign == $campaign->ID ) {
            // Get the combined total of the old & new campaigns.
            $amount = charitable_get_table( 'campaign_donations' )->get_campaign_donated_amount( [ $new_campaign, $old_campaign ] );

            // Subtract the goal of the old campaign.
            $amount -= $old_goal;
        } elseif ( $old_campaign == $campaign->ID ) {
            $amount = $amount > $old_goal ? $old_goal : $amount;
        }

        if ( $sanitize ) {
            $amount = charitable_sanitize_amount( $amount );
        }

        return $amount;
    },
    10,
    3
);