How do I log MySQL errors to a file or database in PHP?

Logging errors is crucial for debugging and monitoring. You can log errors to a file using PHP's file writing functions (`file_put_contents()`, `fwrite()`). Include details like the timestamp, the SQL query that caused the error, and the error message itself. Alternatively, you can log errors to a separate database table. This allows for easier searching and analysis of error data. Consider using a dedicated logging library for more advanced features like log rotation and different log levels.