title: Downloaded Excel File cannot be open properly using PHPExcel
body: <p>In my project, I make exporting data to excel 2007 using PhpExcel 1.8.0.
  I tried it as the followings code. </p>
  
  <p>Excel File is saved in Client side but when I open the downloaded Excel file, I get the warning message "<strong>We found a problem with some content in 'filename.xlsx'. Do you want us to try to recover as much as we can? If you trust the source of this workbook, click Yes.</strong>"</p>
  
  <p>Could anyone help me to solve this problem. Thanks in advance!!!</p>
  
  <pre><code>$_read_sheet = EXCEL_SHEET_PATH . "template.xlsx";
  $ym_dirname = date("Y") . '_' . date("m");
  $output_file_name = "MyFile_" . date("YmdHis", time());
  $output_file = $output_file_name . ".xlsx";
  $output_file = mb_convert_encoding($output_file, "SJIS", "UTF-8");
  $reader = PHPExcel_IOFactory::createReader('Excel2007');
  $xl = $reader-&gt;load("$_read_sheet");
  $xl-&gt;setActiveSheetIndex(0);
  $sheet = $xl-&gt;getActiveSheet();
  $sheet-&gt;setCellValue("AV3", date("Y/m/d"));
  // redirect output to client browser
  header('Content-Type: application/vnd.openxmlformatsofficedocument.spreadsheetml.sheet');
  header('Content-Disposition: attachment;filename="'.$output_file.'"');
  header('Cache-Control: max-age=0');
  $writer = PHPExcel_IOFactory::createWriter($xl, 'Excel2007');
  // Write file to the browser
  $writer-&gt;save('php://output');
  </code></pre>