使用Phpspreadsheet从数据库下载Excel工作表时出错

Question

I am trying to fetch records from a database and then download these records as an excel sheet. It downloads but when I open the excel sheet, I see html codes and some gibberish. I don't even see the records at all. I don't know where the problem is coming from. Here is my php code:

use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Writer\Xlsx;

     $res = $call->fetchRecords();
        $num = $res->num_rows;

        if($num > 0){

            $spreadsheet = new Spreadsheet();
            $sheet = $spreadsheet->getActiveSheet();
            $sheet->setTitle('records');

            $i = 1;
            while($row = mysqli_fetch_assoc($res)){
                $sheet->setCellValue('A'.$i, $row['date']);
                $sheet->setCellValue('B'.$i, $row['Reference']);
                $sheet->setCellValue('C'.$i, $row['amount']);
                $i++;
            }

            $writer = new Xlsx($spreadsheet);
            if(isset($writer)){
                header('Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
                header('Content-Disposition: attachment;filename="pending_transactions.xlsx"');
                header('Cache-Control: max-age=0');
                header('Content-Transfer-Encoding: binary');
                header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT');
                header('Cache-Control: cache, must-revalidate');
                header('Pragma: public');
                $writer->save('php://output');
            }

        }