Change XML header

add_rss_to_xml_header.php

<?php

// With the below filter; wppfm_custom_xml_header you can change the xml header of a custom XML feed 
// make sure you close it with the wppfm_custom_xml_footer filter.

function wppfm_add_rss_to_xml_header( $title ) {
	return '<?xml version="1.0" encoding="UTF-8" ?>/r
			<rss version="2.0"><products>/r';

}

add_filter( 'wppfm_custom_xml_header', 'wppfm_add_rss_to_xml_header' );

function wppfm_add_rss_to_xml_footer( $title ) {
	return '</products></rss>';
}

add_filter( 'wppfm_custom_xml_footer', 'wppfm_add_rss_to_xml_footer' );