<style> .str{ mso-number-format:\@; } </style>
<?php
$file="test.xls";
$test="<style> .str{ mso-number-format:\@; } </style>
<table border=1><tr><td>Cell 1</td><td class='str'>268435459916429000 </td></tr></table>";
header("Content-type: application/vnd.ms-excel");
header("Content-Disposition: attachment; filename=$file");
echo $test;
?>
: