PHP EXCEL export시 시트 이름 지정하여 여러 시트에 데이터 쓰기

<?
$today=date('Ymd');
$file_name=$today.".xls"; //저장할 파일이름
header( "Content-type: application/octet-stream;charset=KSC5601");
header( "Content-Disposition: attachment; filename=$file_name" );
header("Content-Type: application/ms-excel");
header("Pragma: no-cache");
header("Expires: 0");
//header( "Content-Description: PHP4 Generated Data" ); 
echo '<?xml version="1.0" encoding="EUC-KR"?>'; 
?>
<Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet" 
 xmlns:o="urn:schemas-microsoft-com:office:office" 
 xmlns:x="urn:schemas-microsoft-com:office:excel" 
 xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" 
 xmlns:html="http://www.w3.org/TR/REC-html40">  
 <Worksheet ss:Name="aa"> 
  <Table> 
    <Row> 
    <Cell><Data ss:Type="String">a11</Data></Cell> 
    <Cell><Data ss:Type="String">a12</Data></Cell> 
    <Cell><Data ss:Type="String">a13</Data></Cell> 
    </Row> 
    <Row> 
    <Cell><Data ss:Type="String">a21</Data></Cell> 
    <Cell><Data ss:Type="String">a22</Data></Cell> 
    <Cell><Data ss:Type="String">a23</Data></Cell> 
    </Row> 
  </Table> 
 </Worksheet> 
 <Worksheet ss:Name="bb"> 
  <Table> 
    <Row> 
    <Cell><Data ss:Type="String">b11</Data></Cell> 
    <Cell><Data ss:Type="String">b12</Data></Cell> 
    <Cell><Data ss:Type="String">b13</Data></Cell> 
    </Row> 
    <Row> 
    <Cell><Data ss:Type="String">b21</Data></Cell> 
    <Cell><Data ss:Type="String">b22</Data></Cell> 
    <Cell><Data ss:Type="String">b23</Data></Cell> 
    </Row> 
  </Table> 
 </Worksheet> 
</Workbook>