通过php使用ezpdf将数据从mysql导出到pdf的未定义变量会话

通过php使用ezpdf将数据从mysql导出到pdf时遇到问题。问题是，在执行文件时，总是说未定义的变量$sesen_uk。但当我将where条件更改为手动类似生产时，就没有问题了。

<?php
require 'config.php';
require 'class.ezpdf.php';
$sesen_uk     = $_SESSION['unitkerja'];
$sql  = mysqli_query($conn, "SELECT * FROM arsip WHERE lokasi = '$sesen_uk' ");
$i    = 1;
if (mysqli_num_rows($sql) > 0) // Menampilkan data apabila ada datanya
{
  while($tampil = mysqli_fetch_array($sql)) 
  {
    $data[$i] = array ( 'No. Urut'      => $i,
                    'No. Surat'     => $tampil['no_surat'],
                    'Uraian'        => $tampil['uraian'],
                    'Lokasi'        => $tampil['lokasi'],
                  );
            $i++;
  }
$pdf = new Cezpdf('A4','portrait');
$pdf->ezSetCmMargins(1, 3, 3, 3);
$pdf->addJpegFromFile('images/logo.jpg',45,758,40);
$pdf->ezText('Monthly Report', 17, array('justification' => 'center'));
$pdf->ezSetDy(-10);
$pdf->ezSetDy(-10);
$pdf->line(31, 755, 565, 755);
$pdf->ezTable($data, '', '', '');
$pdf->ezSetDy(-50);
$pdf->ezStartPageNumbers(564, 20, 8);
$pdf->ezStream();
}  
  else
  {
    echo '<script>alert("No data");location.replace("report.php")</script>';
  }
?>

任何帮助都会很好，谢谢。