Upgrade PHP 5.4 Web App

I need my existing PHP 5.4 web application brought up to PHP 8.3/+.. The system handles user authentication and some fairly involved data-processing routines, so every login flow, session check, and calculation must continue to behave identically after the upgrade... (Budget: $10 - $30 USD, Jobs: Backend Development, Database Management, JavaScript, MySQL, PHP, Software Architecture, Web Application, Web Development)