PHP: PDO + CSV export not downloading (headers issue?)

Question

I'm having a hard time making my CSV export function work.
I've found around plenty of examples using mysqli_* functions, but I'm actually using PDOs so I had to adapt some questions/answers to my needs.
From what I can see, I'm properly parsing and writing data to the *.csv file, but at the end I don't simply get any "Download" modal from the browser.
Again, looking around, I've understood I may have some kind of problem with my headers, so I'm asking for your help.

This is my PHP function snippet:

function downloadAsCSV($dsn, $dbUser, $dbPsw, $options) {
    // New PDO connection.
    $pdo = pdoConnect($dsn, $dbUser, $dbPsw, $options);

    $query = ... // Not going to write it down.

    // Let's query the database, ...
    $stmt = $pdo->prepare($query);

    // Setting up the query variables here...
    [...]
    // Executing the statement...
    $stmt->execute();

    // Headers.
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    header('Content-Description: File Transfer');
    header("Content-Type: text/csv");
    header("Content-Disposition: attachment; filename='export.csv'");
    header("Pragma: no-cache");
    header("Expires: 0");

    // Let's open tbe "*.csv" file where we'll save the results.
    $fp = fopen("php://output", "w");

    if ($fp) {
        $first_row = $stmt->fetch(PDO::FETCH_ASSOC);
        $headers = array_keys($first_row);
        fputcsv($fp, array_values($first_row));
        fputcsv($fp, $headers);

        // ... fetch the results...
        $workpiecesArray = $stmt->fetchAll();

        // ... and, finally, export them.
        foreach ($workpiecesArray as $workpiece) {
            fputcsv($fp, array_values($workpiece));
        }
    }

    fclose($fp);

    // Closing the connection.
    $stmt = null;
    $pdo = null;
}

function mysqli_field_name($result, $field_offset) {
    $properties = mysqli_fetch_field_direct($result, $field_offset);

    return is_object($properties) ? $properties->name : null;
}

I've taken inspiration to write the table column titles from this answer.

Answer 1

What you need to do is to print the file, which you're not doing right now. See this example using readfile: http://php.net/manual/en/function.header.php#example-5554

Simply put:

1.Replace

$fp = fopen("php://output", "w");

with

$tmpfname = tempnam("/", "");
$fp = fopen($tmpfname, "w");

2.Replace

fclose($fp);

with

fclose($fp);
readfile($tmpfname);
unlink($tmpfname);

and this will work