I want to build a full-featured UPI payment gateway website in PHP with Tailwind css, Lexend and Lucid for frontend styling. The core purpose of this website is to offer the world’s most affordable and free UPI payment service with zero setup charges, no monthly fees, and a free plan with unlimited transactions accepted, and very small per-transaction fees (lower than existing gateways like Razorpay or Instamojo).

The website should have:

1. Landing Page (index.php):
   This should have a modern hero section with a bold headline such as "Accept UPI Payments Instantly — No Setup, No Fees" and a subheadline like "Start collecting payments from anyone via UPI QR or links in seconds."
   Include an eye-catching CTA (Call-To-Action) button — "Start for Free", which scrolls down to a section where users can generate their first payment request instantly. The layout must be responsive, clean, and minimalist, using Bootstrap for fast UI development. Add sections like Features, Pricing, FAQs, and Contact Us.

2. Instant Payment Page (pay.php):
   When a user enters a name and amount, it should generate a UPI payment link and QR code. The page should show the QR and the link, along with a live status checker (JavaScript polling) that monitors for payment confirmation every few seconds.
   No transaction ID input should be required by the user — the backend should automatically detect successful payments by reading new incoming emails using PHP IMAP (the email address should receive real-time UPI alerts from the bank or UPI app). If a match is found (same amount, same user name, recent time), it should display a “Payment Received” success message instantly.

3. PHP Backend API (check.php):
   This file should run in the background (called via AJAX from pay.php) and use PHP's IMAP extension to monitor new Gmail inbox emails, matching recent emails that mention successful payments with the required amount. Once matched, the script should return a success JSON response so the frontend can show confirmation automatically.

4. User Login/Signup & Dashboard (optional):
   If you want to allow merchants to create an account and manage payments, include a MySQL-backed user system with features like:

   • Account Dashboard

   • Payment History

   • Custom UPI QR Generator

   • Referral System or Reseller Panel

5. Database (MySQL):
   Set up a basic database with tables like:

   • users (id, email, password, name, upi_id, plan, created_at)

   • transactions (id, user_id, amount, status, received_at, reference)

   • plans (id, name, monthly_fee, txn_fee_percent, features)

6. Plans & Pricing Page:
   Design a clear pricing page with one Free Plan (₹0/month, unlimited usage, optional “convenience fee” for customer) and one or two Pro Plans (e.g., ₹49/month with lower per-transaction fees or priority processing). Clearly show comparisons and allow upgrade via Razorpay or manual payment.

7. Tech Used:

   • PHP 8.x

   • Tailwind CSS

   • Gmail IMAP integration

   • PHP QRCode library

   • MySQL for storing user data

   • JavaScript + jQuery for polling & frontend actions

8. Security & Verification:
   The IMAP integration must connect using a Gmail App Password. Don’t hardcode credentials — store them securely. Sanitize all user inputs and avoid email flooding by checking recent timestamps (e.g., match within last 2–3 minutes).