1. Definitely get a PHP IDE: https://www.jetbrains.com/phpstorm/
2. Get a step debugger and hook it in: https://xdebug.org/
3. Fix the code style: https://github.com/FriendsOfPHP/PHP-CS-Fixer
4. Run the code through some static analysis tools https://phpstan.org/ https://psalm.dev/
5. Upgrade the code with an AST fixer (might help you update to newer PHP version): https://github.com/rectorphp/rector
6. Add tests to it: https://phpunit.de/
7. Run mutation tests: https://infection.github.io/
The "0 budget" is rough, though.