WordPress PHP Upgrade Fix Involving Older Custom Code

I need an experienced WordPress developer to fix custom PHP code on my site. The site was upgraded from PHP 7.3 to a newer version, which broke some custom functionalities.

Key Requirements:
- Fix third-party integrations, especially Stripe payment gateway and custom commission calculations.
- Work on existing custom code to restore functionality, with possible improvements in designs and layouts.

Ideal Skills & Experience:
- Strong knowledge of WordPress and PHP
- Experience with Stripe and custom integrations
- Ability to troubleshoot and fix legacy code

I will provide:
- A working version of the site (not upgraded)
- A non-working version of the site (upgraded) for comparison

Your expertise will ensure a smooth transition to the updated PHP version without losing any custom functionalities.