I am really not into php, but here is what a small search lead me to.
In order to remove the doctype associated with your root
<svg> element, you would have to import this element inside a new, clean, DOMDocument.
From there, you'll be able to call this DOMDocument's ::saveXML(DOMNode) method, and the original noise will be gone.
$xml = simplexml_load_file($file); $cleanDom = new DOMDocument(); $node = dom_import_simplexml($xml); $clone = $cleanDom -> importNode($node, true); $cleanMarkup = $cleanDom -> saveXML($clone);