The simpliest (and probably the fastest) way to strip an XML declaration (<?xml version="1.0" ... ?>) out of output document, is to output child nodes of DOMDocument separately:

<?php

$document = new DOMDocument();
$document->load('/some/file.xml');

// this will also output doctype and comments at top level
foreach($document->childNodes as $node)
    $result .= $document->saveXML($node)."\n";

echo $result;

?>

This might be userful when dealing with browser compatibility issues, for example, well known problem with valid XHTML in IE6.