Fix PHP/MySQL Bug in Custom Admin Panel (Urgent)

We have a custom-built PHP web application (no framework, plain PHP + MySQL) that handles order management for our small business. Recently, one of our forms started throwing an error when saving new entries — it seems related to a SQL query or possibly a session timeout issue. What's broken: When submitting the "Add order" form, the page shows a blank white screen or a MySQL error This started happening after we added a new field to the database Sometimes it works, sometimes it doesn't (inconsistent) What we need: Diagnose the root cause (likely a query bug or data type mismatch) Fix it without breaking other parts of the panel Test on staging before pushing live Requirements: Strong PHP + MySQL debugging experience Comfortable working with legacy/undocumented code Can start immediately, quick turnaround preferred Please share similar bug-fix work you've done before applying Budget fixed Timeline: ASAP, within 24–48 hours