Passing XML Doc's

Hi there Is there anyway of passing a xml document to a php page as raw data. What I want to do is eg. send a page a xml document and recieve a response ( in xml ) from that page. I could eg get the result using fopen( 'http://url.com/xml.php' , 'r' ) but how do i send it any data without using for eg. fopen( " http://url.com/xml.php?xml=xml_string_document ", 'r' ) Any help would be VERY helpfull thanks Niel