znalezione na php.net
Niestety potrzebujesz do tego Windowsa i Excela.
Nie sprawdzałem, czy to działa

<?php
// starting excel
$excel = new COM
("excel.application") or
die("Unable to instanciate excel"); print "Loaded excel, version {$excel->Version}\n";
//bring it to front
#$excel->Visible = 1;//NOT
//dont want alerts ... run silent
$excel->DisplayAlerts = 0;
//open document
$excel->Workbooks->Open("C:\\mydir\\myfile.xls");
//XlFileFormat.xlcsv file format is 6
//saveas command (file,format ......)
$excel->Workbooks[1]->SaveAs("c:\\mydir\\myfile.csv",6);
//closing excel
$excel->Quit();
//free the object
$excel->Release();
$excel = null;
?>