<?php
$db_con = mysqli_connect("localhost", "username", "password", "database");
$result = $db_con->query('SELECT * FROM some_table');
if (!$result) die('Couldn\'t fetch records');
$num_fields = mysqli_num_fields($result);
$headers = array();
while ($fieldinfo = mysqli_fetch_field($result)) {
    $headers[] = $fieldinfo->name;
}
$fp = fopen('php://output', 'w');
if ($fp && $result) {
    header('Content-Type: text/csv');
    header('Content-Disposition: attachment; filename="export.csv"');
    header('Pragma: no-cache');
    header('Expires: 0');
    fputcsv($fp, $headers);
    while ($row = $result->fetch_array(MYSQLI_NUM)) {
        fputcsv($fp, array_values($row));
    }
    die;
}
?>