BBPS API Integration PHP & Kotlin

I already run a recharge portal where prepaid, DTH, and gift-card transactions work through existing APIs. The next step is adding full Bharat Bill Payment System (BBPS) functionality across both channels at the same time—our PHP-based website and the Kotlin Android application.

Here’s what I need you to handle:

• End-to-end integration of the approved BBPS API, covering authentication, biller catalogue sync, bill fetch, payment, callback handling, and transaction logs.
• Common business logic where possible, with separate presentation layers for web (PHP) and mobile (Kotlin).
• Error handling that gracefully falls back to existing services if the BBPS gateway is temporarily unavailable.
• Clear, well-documented code and setup instructions so my in-house team can maintain or extend the feature later.

Acceptance will be based on:
1. Successful test payments to at least three different biller categories on both platforms.
2. Consistent transaction status updates reflected in our admin panel.
3. Handover of source code and post-deployment walkthrough.

All current server endpoints, keys, and UI screens are ready; you’ll receive them once we start. If you’ve previously worked with BBPS, that experience will come in handy, but solid PHP, Kotlin, REST/JSON, and payment-gateway security practices are what really matter.