如何使用tcpdf输出mysql表？

Question

我尝试使用mysql将tcpdf表打印到PDF，但输出只是空白（除了页眉和页脚）。我正在做的是返回fetchdata()函数中的所有内容。我究竟做错了什么？我不明白为什么表没有输出，因为它可以输出页眉和页脚到PDF。非常感谢您的帮助。

    <?php
        session_start();
        $date= $_SESSION["date"];
        function fetchdata($date)
        {
            $output='';
            $output .= "<p>" . date_format($date, "F j, Y") . " Payroll Report</p>";
            $output .= "<table>";

            $output .= "<th>Employee Name</th>";
            $output .= "<th>Basic Pay</th>";
            $output .= "<th>Rate/Day</th>";
            $output .= "<th>Rate/Hour</th>";
            $output .= "<th>Overtime</th>";
            $output .= "<th>Allowance</th>";
            $output .= "<th>Commissions</th>";
            $output .= "<th>Housing Fee</th>";
            $output .= "<th>Cash Advances</th>";
            $output .= "<th>Loans</th>";
            $output .= "<th>Coop Share</th>";
            $output .= "<th>SSS</th>";
            $output .= "<th>Philhealth</th>";
            $output .= "<th>Pagibig</th>";
            $output .= "<th>Personal Gas</th>";
            $output .= "<th>Absences</th>";
            $output .= "<th>Withholding Tax</th>";
            $output .= "<th>Salary Differential</th>";
            $output .= "<th>Total</th>";
            $output .= "<tr>";
            require('functions.php');
            require('connecttoserver.php');
            $sql = 'SELECT * FROM employees ORDER BY employee_id ASC';
            $result = $conn->query($sql);
            ob_start();
            while($row = $result->fetch_assoc() {
                $output .= "<tr>";
                $output .= "<td>" . $row['name'] . "</td>";
                $basic_pay=basic_pay($row["employee_id"],$date);
                $output .= "<td>" . $basic_pay . "</td>";
                $rate_per_day=rate_per_day($row["employee_id"],$date);
                $output .= "<td>" . $rate_per_day . "</td>";
                $rate_per_hour=rate_per_hour($row["employee_id"],$date);
                $output .= "<td>" . $rate_per_hour . "</td>";
                $overtime=overtime($row["employee_id"],$date);
                $output .= "<td>" . $overtime . "</td>";
                $allowance=allowance($row["employee_id"],$date);
                $output .= "<td>" . $allowance . "</td>";
                $commissions=commissions($row["employee_id"],$date);
                $output .= "<td>" . $commissions . "</td>";
                $housing_fee=housing_fee($row["employee_id"],$date);
                $output .= "<td>" . $housing_fee . "</td>";
                $cash_advances=cash_advances($row["employee_id"],$date);
                $output .= "<td>" . $cash_advances . "</td>";
                $loans=loans($row["employee_id"],$date);
                $output .= "<td>" . $loans . "</td>";
                $coop_share=coop_share($row["employee_id"],$date);
                $output .= "<td>" . $coop_share . "</td>";
                $sss=sss($row["employee_id"],$date);
                $output .= "<td>" . $sss . "</td>";
                $philhealth=philhealth($row["employee_id"],$date);
                $output .= "<td>" . $philhealth . "</td>";
                $pagibig=pagibig($row["employee_id"],$date);
                $output .= "<td>" . $pagibig . "</td>";
                $personal_gas=personal_gas($row["employee_id"],$date);
                $output .= "<td>" . $personal_gas . "</td>";
                $absences=absences($row["employee_id"],$date);
                $output .= "<td>" . $absences . "</td>";
                $withholding_tax=withholding_tax($row["employee_id"],$date);
                $output .= "<td>" . $withholding_tax . "</td>";
                $salary_differential = salary_differential($row["employee_id"],$date);
                $output .= "<td>" . $salary_differential . "</td>";
                $total = $basic_pay + $overtime + $allowance + $commissions - $housing_fee - $coop_share - $sss - $philhealth - $pagibig - $cash_advances -$loans - $personal_gas - $absences - $withholding_tax + $salary_differential;
                $output .= "<td>" . $total . "</td>";
                $output .= "</tr>"; 
            }
            $output .= "</tr>";
            $output .= "</table>";
            return $output;
            $conn->close();
       }
    // Include the main TCPDF library (search for installation path).
    require_once(dirname(dirname(__FILE__)) . '/tcpdf/tcpdf.php');
    // create new PDF document
    $pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);

    // set document information
    $pdf->SetAuthor('CCJD Camarao Ventures, Inc.');
    $pdf->SetTitle('Payroll Report: ' . $date->format('m-d-Y'));

    // set default header data
    $pdf->SetHeaderData('logo.jpg', PDF_HEADER_LOGO_WIDTH, $companyname, '', $address);

    // set header and footer fonts
    $pdf->setHeaderFont(Array(PDF_FONT_NAME_MAIN, '', PDF_FONT_SIZE_MAIN));
    $pdf->setFooterFont(Array(PDF_FONT_NAME_DATA, '', PDF_FONT_SIZE_DATA));

    // set default monospaced font
    $pdf->SetDefaultMonospacedFont(PDF_FONT_MONOSPACED);

    // set margins
    $pdf->SetMargins(PDF_MARGIN_LEFT, PDF_MARGIN_TOP, PDF_MARGIN_RIGHT);
    $pdf->SetHeaderMargin(PDF_MARGIN_HEADER);
    $pdf->SetFooterMargin(PDF_MARGIN_FOOTER);

    // set auto page breaks
    $pdf->SetAutoPageBreak(TRUE, PDF_MARGIN_BOTTOM);

    // set image scale factor
    $pdf->setImageScale(PDF_IMAGE_SCALE_RATIO);

    // add a page
    $pdf->AddPage('L', 'LETTER');

    $html=fetchdata($date);
    $html = ob_get_contents();
    $pdf->writeHTML($html, true, false, true, false, '');
    // reset pointer to the last page
    echo $pdf->lastPage();

    ob_end_clean();
    //Close and output PDF document
    echo $pdf->Output('PayrollReport $date', 'I');
?>