You may want to try setting setting the Content Type attributes in the header. You can do this by putting this in your php file (before outputting anything mind you):
header("Content-type: application/x-msexcel");
*Also add this so it saves it as the right filename:
header("Content-Disposition: inline; filename=filename.extension");
For me when this is set it automaticaly asks you to save the document instead of displaying the information in the browser.