Php 000webhost中生成的csv文件未下载，而是在网页中打印结果。有什么解决办法吗？

标题

$file_name = 'data.csv'; 

sql查询

    header("Content-Description: File Transfer");
    header("Content-Disposition: attachment; filename=$file_name");
    header("Content-Type: application/csv;");
    $file = fopen('php://output', 'w');
    $header = array("value");
    fputcsv($file, $header);

$query=“从日期>=”的数据中选择*”$\u POST[“开始日期”]”和日期
    $query = "SELECT * FROM data WHERE date >= '".$_POST["start_date"]."' AND date <= '".$_POST["end_date"]."' ORDER BY packet_id ASC";
    $statement = $connect->prepare($query);
    $statement->execute();
    $result = $statement->fetchAll();
    foreach($result as $row) {
        $data = array();
        $data[] = $row["value"];   
        fputcsv($file, $data);
    }
    fclose($file);
    exit;
    }