generated XML with PHP extension not read

I am using a PHP file to generate my XML file.
I DO have as my first line (no whitespace) the php command header(‘Content-type: application/xml’); and viewing the page information it is being interpreted by the browser as an XML file without difficulty.
If I simply save the resulting file with the .xml extension instead of the .php it also works fine, so the XML is well formed.
I believe the problem is in the saving of the “XML file path” tab in the “WP Supersized source of images” panel, as when I insert the correct path and file name “uploads/subgallery/xml.php” the panel, upon saving, reverts to the “Custom dir” and lists the *directory* as “http://localhost/wp-content/uploads/subgallery/xml.php” and the page shows the error background with the message “WP Supersized cannot find your directory or file please check that it exists”.

http://wordpress.org/extend/plugins/wp-supersized/