How can one troubleshoot the error message "Von Excel wurde unlesbarer Inhalt in ... gefunden" when opening an Excel file generated using PHP?

The error message "Von Excel wurde unlesbarer Inhalt in ..." typically indicates that the Excel file generated using PHP may contain invalid characters or formatting that Excel cannot read properly. To troubleshoot this issue, try encoding the Excel file in UTF-8 format before saving it, as Excel tends to have better compatibility with this encoding.

// Generate Excel file
$spreadsheet = new PhpOffice\PhpSpreadsheet\Spreadsheet();
// Add data to the spreadsheet
// ...

// Set UTF-8 encoding
$writer = PhpOffice\PhpSpreadsheet\IOFactory::createWriter($spreadsheet, 'Xlsx');
$writer->setUseBOM(true); // Add BOM to mark as UTF-8
$writer->save('output.xlsx');