add comments

export_offer.php

@@ -1,6 +1,8 @@
 <?php
     include 'functions.php';
+    //add an XML header to the output
     header('Content-type: text/xml');
+    //prompt the browser to download the output as a file called offer.xml
     header('Content-Disposition: attachment; filename="offer.xml"');
     export_offer();
 ?>