This blog explains, how to create a CSV file using PHP and how to download the file instead of displaying it.

<?php
// Creates a new csv file and store it in tmp directory
$new_csv = fopen('/tmp/report.csv', 'w');
fputcsv($new_csv, $row);
fclose($new_csv);

// output headers so that the file is downloaded rather than displayed
header("Content-type: text/csv");
header("Content-disposition: attachment; filename = report.csv");
readfile("/tmp/report.csv");
?>

This code tells the browser that it is generating a CSV file and that should be offered for download, rather than displayed in the browser.

header("Content-type: text/csv");

The MIME type of the content is text/csv, the official MIME type for CSV files.

header("Content-disposition: attachment; filename=report.csv");

header to supply a recommended filename and force the browser to display the save dialog.

readfile("/tmp/report.csv");

readfile — Outputs a file.