PHP, Excel, and tab delimited files question

In many of our apps we save tab delimited text files to be later opened from the browser using the following set of headers; header("Content-Type: application/vnd.ms-excel"); header("Content-Disposition: inline; filename=\"excel.xls\""); header("Expires: 0"); header("Cache-Control: must-revalidate post-check=0 pre-check=0"); However, fields that start with 0 (zero) generally drop the zero so that 0201 becomes 201 (which is not correct for our purposes). Does anyone know a quick way to specify in the header to make it text only? TIA!