WooCommerce/dynamic properties: associating arbitrary values with individual entity instances.

entity-arb-data-maps.php

<?php
/**
 * Simple approach showing a structured approach to associating arbitrary, ephemeral
 * (non-persisted) data with individual product objects and other entities.
 * 
 * Benefits include avoidance of collisions between two plugins using the same key,
 * the range of checks that can be applied to stored values, etc.
 *
 * @link https://github.com/woocommerce/woocommerce/issues/45286
 */

namespace My_Plugin {
	use WeakMap;
	use SplObjectStorage;

	function internal_costs() {
		static $map;

		if ( empty( $map ) ) {
			$map = class_exists( WeakMap::class ) ? new WeakMap : new SplObjectStorage;
		}

		return $map;
	}
}

namespace {
	$product        = wc_get_product( 12345 );
	$internal_costs = My_Plugin\internal_costs();

	$internal_costs[ $product ] = 123;
	print $internal_costs[ $product ];
}