php:pdo+csv导出未下载(标题问题?)

我很难让我的csv导出功能工作。
我已经找到了很多使用mysqli_u*函数的例子,但实际上我使用的是PDO,所以我必须根据需要调整一些问题/答案。
对于我所看到的,我正在正确地解析数据并将其写入到*.csv文件中,但最后我不会简单地从浏览器中获得任何“下载”模式。
再次,环顾四周,我知道我的头可能有点问题,所以我需要你的帮助。

这是我的PHP函数片段:

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;
}

我从中得到灵感,写下了表列的标题。 this answer .