Add an array_sum utility method to enhance PHP 8.3+ compatibility

[coreymckrill]

Changes proposed in this Pull Request:

In #40660 several people are reporting warnings such as the following when running on PHP 8.3:

PHP Warning: array_sum(): Addition is not supported on type string in /home/public_html/wp-content/plugins/woocommerce/includes/rest-api/Controllers/Version1/class-wc-rest-orders-v1-controller.php on line 271

This seems to happen when for some reason a non-numeric value makes it into an array that should be all integers and floats, possibly due to legacy data that hasn't been sanitized correctly or extensions that inject incorrect data types.

Rather than fixing each case of this separately, this PR introduces a new array_sum method to the NumberUtil class that takes a similar approach to the round method that is already in that class. It acts as a wrapper around the built-in array_sum, while adding a sanitization step to ensure that all the values in the array are numeric.

Note that there are over 100 uses of array_sum in the WooCommerce codebase. Rather than updating every usage to the new method, which would cause this PR to have dozens of changed files and be difficult to test, as well as troubleshoot if something goes wrong, this simply fixes the usage cases that have already been reported to be problematic, as well as a few that seem especially likely to have the same issue.

Fixes #40660

How to test the changes in this Pull Request:

Code review, and executing the test suite under PHP 8.3, is probably the best way to assess this change. Some manual steps that can be performed for added confidence:

1. Make sure your test site is running on PHP 8.3, and that your site has WP_DEBUG_LOG set to true.
2. Set up a tax rate, shipping zone, and shipping method. Make sure the shipping method is marked as taxable.
3. Create an order. Add billing and shipping details that will make the order qualify for the tax rate and the shipping zone.
4. Add items to the order, including shipping.
5. Add this code snippet to your site. This will inject extra, invalid values into the shipping taxes when you make a REST API request:

   add_filter(
   	'woocommerce_order_item_get_taxes',
   	function ( $value ) {
   		$value['total'][] = '';
   		$value['total'][] = 'asdf';
   		$value['total'][] = '1,3';
   
   		return $value;
   	}
   );

6. Make the following request to the REST API (note that it's using v1 of the API, not the current v3):

   curl --request GET \
   	--url 'http://localhost:8888/wp-json/wc/v1/orders/53?_fields=id%2Cshipping_lines'
   

7. In the response, under the shipping_lines property and the taxes property within that, you should see the shipping tax you added to the order, plus the extra values injected by the code snippet. You should also see "total_tax", which should contain the correct sum of the valid tax items.
8. Now check your log file, enabled by WP_DEBUG_LOG. This is generally found at wp-content/debug.log. Ideally the file won't be there, because no PHP warning was logged. But if the file exists, check to make sure it does not have an entry like the one listed at the top of this PR.

[github-actions]

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[github-actions]

Hi @Konamiman,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[github-actions]

Hi @Konamiman,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions