Our production system still lives on CodeIgniter 1.7.2 with PHP 5.3. The task is to refactor the entire code-base so it runs flawlessly on CodeIgniter 4.7 and PHP 8.x, without losing a single feature. Current feature set • User authentication and management • Five-step e-commerce flow with PayPal IPN • 12–15 admin modules that handle CRUD on multiple master tables Database considerations The MySQL schema and data have to move forward as well. I need every table, relation and record preserved, indexed sensibly for modern workloads, and fully compatible with CI 4’s models. Data integrity, consistency and measurable performance gains are all mandatory. Scope of work 1. Re-architect legacy controllers, models and views into CI 4’s MVC structure, namespacing and routing. 2. Replace deprecated PHP calls and libraries with PHP 8-ready equivalents. 3. Re-implement the PayPal IPN callback in CI 4, keeping the five-step order process intact. 4. Design and run migration scripts that port the existing database while keeping production data live and intact. 5. Deliver unit and functional tests that cover the migrated authentication, checkout and admin modules. Acceptance criteria • Application installs via Composer, passes CI 4’s built-in tests and boots on PHP 8.x. • All existing user accounts, orders and admin records are present and functional in the new environment. • End-to-end checkout completes and receives valid PayPal IPN confirmation. • Page load times and query latency meet or beat current production figures. • A step-by-step deployment guide lets us reproduce the migration on staging and production. Please base the work on a private Git repository so I can review commits in real time.