PHP Code Upgrade to 8.1

I need my custom PHP website, currently running on PHP 7.4, updated to PHP 8.1 or above.

Requirements:
- Update existing code to be compatible with PHP 8.1
- No new features or changes to the current functionality
- No automated tests are currently in place

Ideal Skills and Experience:
- Strong expertise in PHP, especially versions 7.4 and 8.1
- Experience with custom PHP applications
- Ability to thoroughly test the updated code to ensure existing functionality is maintained

Please provide your approach and estimated time for completion.