Fix PHP Error on Local XAMPP Site

My website, developed with a colleague, is experiencing a PHP error on startup. I need help diagnosing and troubleshooting this problem. The site is currently stored on my local machine and has not yet been hosted.

Ideal skills and experience:
- Strong PHP development skills
- Experience with XAMPP and local development environments
- Ability to troubleshoot and debug PHP bugs
- Clear communication skills to explain the problem and its solution