This is just a guess, it depends on their interface of course.
The basic idea is that PHP usually outputs HTML, you know html tags. So instead of echo-ing out html tags all you have to do is echo out a xml file. But it sounds like that they want you do make a specific input field called 'xml' to hold the xml value.
Reading between your post this is what I understand they are asking you to do. They have a interface that you will call, lets call it 'interface.php'. You make a page called 'test.php' on this page you have this code:
<html>
<body>
<form method="POST" action="interface.php">
<?php
//Here you would need to know the XML value to work with there interface
$xmlCommand = ..... ;//Create xml command value
?>
<input type="hidden" name="xml" value="<?=$xmlCommand?>">
<input type="submit" name="btnSubmit" value="View Books">
</form>
</body>
</html>
This would call their interface page with your xml command in the xml field. I would have to understand there interface to give you more help. Hope that gets you thinking in the right direction.
