save the completed form pdf (using xfdf) to the server rather than displaying it to the user

Question

I have successfully created the xfdf output and correctly filled out the pdf.

header ("Content-Type: application/vnd.adobe.xfdf");
print createXFDF($url, $values)

How do I save the completed form pdf (using xfdf) to the server rather than displaying it to the user?

Unfortunately pdftk is not an option.