It's very easy to do using this pear project.
http://pear.php.net/package-info.php?package=Spreadsheet_Excel_Writer
Here is a code sample:
/* Load the object */
require_once 'Spreadsheet/Excel/Writer.php';
/* Create a workbook */
$workbook = new Spreadsheet_Excel_Writer();
/* Send the headers */
$workbook->send('player_ratings.xls');
/* Create the worksheet */
$worksheet =& $workbook->addWorksheet('ratings');
/* Cycle through */
$row = 0;
foreach($d->players as $player)
{
$ratingDesc = $d->ratingPickList->getValueForID($player->rating);
$worksheet->write($row, 0, $player->playerID);
$worksheet->write($row, 1, $player->lastName);
$worksheet->write($row, 2, $player->firstName);
$worksheet->write($row, 3, $player->dob);
$worksheet->write($row, 4, $ratingDesc);
$worksheet->write($row, 5, $player->note);
$row++;
}
/* Send It */
$workbook->close();