How neat!

Here's a little Excel-file writer in PHP; Excel will read these files, but this library won't:

    <?php echo '<?xml version="1.0" encoding="utf-8"?>'; ?>
    <?php echo '<?mso-application progid="Excel.Sheet"?>'; ?>
    <Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet"
        xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" 
        xmlns:o="urn:schemas-microsoft-com:office:office"
        xmlns:x="urn:schemas-microsoft-com:office:excel" 
        xmlns:html="http://www.w3.org/TR/REC-html40">
      <Worksheet ss:Name="Sheet1">
        <?php $data = array(array(4,5,6),array("dude","whatever","m<om")); ?>
        <Table>
          <?php foreach($data as $row): ?>
          <Row>
            <?php foreach($row as $cell): ?>
            <Cell>
              <Data ss:Type="String"><?= htmlentities($cell) ?></Data>
            </Cell>
            <?php endforeach; ?>
          </Row>
          <?php endforeach; ?>
        </Table>
      </Worksheet>
    </Workbook>

They're "technically" Microsoft Office XML-format[1], and not "Excel files", but users don't tend to be amused by programmers being technically right so I tend not to argue about that sort of thing. I only bring it up here in case someone wants to search for a specification on it.

[1]: https://en.wikipedia.org/wiki/Microsoft_Office_XML_formats