I am downloading some data from the database in the form of an Excel file (CSV format)..
I am using the header method to download the CSV file.
My code is as follows...
header("Content-Disposition: attachment; filename=export.csv");
Since we don't know the tables nor the query acts as the source for the csv file and more importantly we don't know the data included in the CSV file, we can't give you accurate answer.
However keep in mind, that CSV is not a standardised format, therefore implementations may differ.
You can get the error message, because the software you are using has a different implementation (for example, it fails to parse multiline values, such as description texts).
Typical problems are:
If you are assembling the CSV content by hand, be sure, that you are doing it in a way what can be handled by the target software.
Test your code (the CSV generation) using simple and short data to be sure, that it is working fine.
The content type of the CSV files is
text/csv according to RFC4180.
PS: Marking your question as "unclear what you are asking", since the provided info is way not enough to solve and narrow down the problem.