Plasmonix/CC-Checker ? reverse-engineered prompt

Build me a simple PHP website that lets me paste in a credit card number and see whether it looks valid or not, with a clean page in the browser and a backend endpoint that does the checking. I want it to run locally in XAMPP just by opening the project in the htdocs folder, so keep setup easy and make sure the homepage works right away.

Please include a basic form, a result area, and a straightforward response from the PHP API file. If Stripe needs to be used, keep it in a safe test style and make the app work without me having to set up anything complicated. Make the layout look decent with the assets folder, and add clear messages for success, failure, and any errors. If you need to look up current docs online, feel free to do that.