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.
Currently if i upgrade its hosting php to 8.XX, the script breaks but it works well on php 5.4

What I expect from you:

• Update the codebase so it runs cleanly under PHP 8, replacing or refactoring any deprecated or removed functions.
• Resolve compatibility issues that might surface in authentication, database interactions, and the data-processing logic.
• Run thorough functional tests to prove that all current features work the same way on the new version.
• Deliver the updated source code along with a brief change log and notes on any server-side tweaks I’ll need to apply.

If you have recent experience migrating sizeable PHP 5.x projects to PHP 8, this should be a straightforward assignment.