If all you want is a simple excel worksheet try this:
header('Content-type: application/excel');
$filename = 'filename.xls';
header('Content-Disposition: attachment; filename='.$filename);
$data = '<html xmlns:x="urn:schemas-microsoft-com:office:excel">
<head>
<!--[if gte mso 9]>
<xml>
<x:ExcelWorkbook>
<x:ExcelWorksheets>
<x:ExcelWorksheet>
<x:Name>Sheet 1</x:Name>
<x:WorksheetOptions>
<x:Print>
<x:ValidPrinterInfo/>
</x:Print>
</x:WorksheetOptions>
</x:ExcelWorksheet>
</x:ExcelWorksheets>
</x:ExcelWorkbook>
</xml>
<![endif]-->
</head>
<body>
<table><tr><td>Cell 1</td><td>Cell 2</td></tr></table>
</body></html>';
echo $data;
The key here is the xml data. This will keep excel from complaining about the file.