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.