Custom Gateway Integration for PHP Site

My PHP-based website is already up and running; the only thing left is to hook it to a Chinese payment gateway so customers can complete their orders without leaving the site. I have the gateway’s API documentation, merchant keys, and test credentials on hand—what I’m missing is the clean, secure code that ties everything together.

Here is what I need from you:

• Build the server-side integration in PHP, following the gateway’s REST/HTTP spec (token creation, order submission, signature generation, callback/notification handling, and refund endpoint).
• Add the corresponding front-end call so the checkout page triggers the gateway’s payment flow smoothly.
• Verify transactions end-to-end in the sandbox, then switch me to production keys once we confirm success.
• Keep the code modular so I can drop future gateways in with minimal changes.
• Provide a brief read-me explaining any config variables, the signature logic, and where to update keys later.

Once the callback returns a confirmed status and the order table updates correctly, the job is done. If you have prior experience with Alipay, WeChat Pay, UnionPay, or similar Chinese platforms in a plain-PHP environment, you’ll fly through this. Let me know your timeline and any questions about the docs, and we can get started right away.