The other php file (which will returns the respmse) will process your information ..like -
<?php
$a = $POST['a'];
$b = $POST['b'];
/Do your processing here and make the rsponse $c/
//Now If it depends on you which type of returns do you want to return, if you want to return simple format or html for mat just echo it
echo $c
OR
//If you want to return xml format. Create your response $c in xml format and set header.
header("Content-Type: application/xml");
echo '<'.'?xml version="1.0"?'.">\n";
echo $c // $c is xml formatted response
?>