Cannot modify header information - headers already sent

[eluser]Sudz[/eluser]
[quote author="Sudhakar Prajapati" date="1304168206"]I got this output : Headers were already sent at : on line : 0[/quote]


Here is my code

Code:

ob_start();
$objWriter->save('php://output');
$file_name = ob_get_contents();
ob_end_clean();
                
zipFilesAndDownload($file_name,'./user_data/leads.zip');

function zipFilesAndDownload($file_name,$archive_file_name)
    {
          //create the object
          $zip = new ZipArchive();
          //create the file and throw the error if unsuccessful
          if ($zip->open($archive_file_name, ZIPARCHIVE::CREATE )!==TRUE) {
            exit("cannot open <$archive_file_name>\n");
              }
          //add each files of $file_name array to archive
                
          $zip->addFromString("leads/export.xls",$file_name);
          $zip->close();
           //then send the headers to force download the zip file
                
        header('Content-Type: application/vnd.ms-excel');
        header("Content-Disposition: attachment;filename=$archive_file_name");
        header('Cache-Control: max-age=0');
        header("Content-Transfer-Encoding: binary");
        header('Pragma: public');
        readfile($archive_file_name);
         exit;
}

What i am missing?