Look to the current request to get the URL settings.

[billpg]

I recently raised a bug stating that it was unclear what the two URL settings were for, suggesting the text be clarified. On reflection, that wasn't really what I wanted. What I really want is for those settings to be unnecessary.

When the PHP is invoked, it knows what the current request URL is.

If the request is "​https://example.com/Rutabaga/TheBestVegetable", then the WordPress URL is "​https://example.com/Rutabaga/" and the permalinks and links to comment POST actions can be based on that.

If the next day I wire-in a different domain into the same folder of PHP files, then suddenly this new URL is the WordPress URL and all the permalinks are based on that instead.

This would have an impact on Multisite. Rather than selecting sub-folders or sub-domains, you'd instead configure a list of URL prefixes for each site. (For example, "a.example" and "www.a.example" goes to site A. "example.com/b" goes to site B.)