Fix PHP Inventory System Errors

I'm looking for an experienced developer to update and fix our browser-based stocktake inventory system built with PHP and MySQL. Recently, after adding new features to the system, we started encountering a 500 page error immediately following the code changes.

Key Requirements:
- Diagnose and resolve the 500 page error
- Review and debug recent code changes related to new feature additions
- Ensure all functionalities, including data saving and stock level accuracy, are working correctly
- Optimize the system for stability and performance

Ideal Skills and Experience:
- Strong expertise in PHP and MySQL
- Experience with debugging and resolving server errors, particularly 500 errors
- Familiarity with inventory management systems
- Ability to quickly understand and modify existing codebases
- Good problem-solving skills and attention to detail

If you have the skills to help us resolve these issues and improve our system, I would love to hear from you.