Writing Excel File Through PHP

I have to generate reports in Excel formats.I send the appropriate content type through header() function of PHP.But I can't get the <td> colors when using css sheets.
Following problem occurs:
1. Image doesn't come.
2.<td> does'nt take css style.
Following is the code snippet:

<?php
header("Content-type: application/vnd.ms-excel");
header("Content-disposition: attachment; filename=pareto_combined.xls");
?>
<link href="../../../extra/sheets/SSheet.css" rel="stylesheet" type="text/css">
<tr colspan=8>
<td height=50> <img src='../../../extra/images/log.jpg'></td>
<td align=right><img src='../../../extra/images/mickey_sup.jpg'></td>
</tr>

<?php
$dateF=$_REQUEST['dateF'];

$dateT=$_REQUEST['dateT'];

$agentID=$_REQUEST['agentID'];
?>
<tr>
<td class='graybg'><?php echo $agentID</td>
<td class='graybg'><?php echo $dateF?></td>
<td class='graybg'><?php echo $dateT?></td>
</tr>