如何在PHP中从XML中删除实体声明

我试图从XML文件中删除<!ENTITY定义而没有成功，我认为通过使用以下代码片段，输出将不包含实体定义的痕迹，但我错了。

我怎样才能达到这个目标?

DOMDocument::loadXML(): xmlns: URI &ns_svg; is not absolute in Entity

旁边没有错误信息

小上下文:

我嵌入一个SVG在另一个，但<!ENTITY给了我各种各样的问题，所以我正在考虑使用LIBXML_NOENT和删除所有的<!ENTITY定义。

PHP:

<?php
header('Content-Type: text/plain');
$str = file_get_contents(dirname(__FILE__) . '/test2.svg');

$document = new DOMDocument();
$document->loadXML($str);
foreach ($document->doctype->entities as $entity) {
    $entity->parentNode->removeChild($entity); // I thought this would remove the <!ENTITY declaration
}
echo $document->saveXML(); // --> I want the XML without <!ENTITY ns_svg "http://www.w3.org/2000/svg"> and <!ENTITY ns_xlink "http://www.w3.org/1999/xlink">

XML:

<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 12.0.1, SVG Export Plug-In . SVG Version: 6.00 Build 51448)  -->
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" [
    <!ENTITY ns_svg "http://www.w3.org/2000/svg">
    <!ENTITY ns_xlink "http://www.w3.org/1999/xlink">
]>
<svg  version="1.1"
     id="Bundesschild" sodipodi:version="0.32" xmlns:cc="http://web.resource.org/cc/" xmlns:sodipodi="http://inkscape.sourceforge.net/DTD/sodipodi-0.dtd" inkscape:version="0.43" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:svg="http://www.w3.org/2000/svg" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape" sodipodi:docbase="D:'Kuvat'Wikipedia" sodipodi:docname="Flag_of_Germany_(state).svg"
     xmlns="&ns_svg;" xmlns:xlink="&ns_xlink;" width="250" height="275" viewBox="0 0 250 275"
     overflow="visible" enable-background="new 0 0 250 275" xml:space="preserve">
<path id="Schild" fill="#FFCE00" stroke="#000000" d="M235.885,2.558c0,0,0,131.825,0,171.735
    c0,54.121-50.504,98.265-112.501,98.265c-61.996,0-112.5-44.144-112.5-98.265c0-39.91,0-171.735,0-171.735H235.885z"/>
</svg>

输出:

<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 12.0.1, SVG Export Plug-In . SVG Version: 6.00 Build 51448)  -->
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" [
<!ENTITY ns_svg "http://www.w3.org/2000/svg"> <!-- Why this is still here???? -->
<!ENTITY ns_xlink "http://www.w3.org/1999/xlink"> <!-- Why this is still here???? -->
]>
<svg xmlns:cc="http://web.resource.org/cc/" xmlns:sodipodi="http://inkscape.sourceforge.net/DTD/sodipodi-0.dtd" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:svg="http://www.w3.org/2000/svg" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape" xmlns="&ns_svg;" xmlns:xlink="&ns_xlink;" version="1.1" id="Bundesschild" sodipodi:version="0.32" inkscape:version="0.43" sodipodi:docbase="D:'Kuvat'Wikipedia" sodipodi:docname="Flag_of_Germany_(state).svg" width="250" height="275" viewBox="0 0 250 275" overflow="visible" enable-background="new 0 0 250 275" xml:space="preserve">
<path id="Schild" fill="#FFCE00" stroke="#000000" d="M235.885,2.558c0,0,0,131.825,0,171.735  c0,54.121-50.504,98.265-112.501,98.265c-61.996,0-112.5-44.144-112.5-98.265c0-39.91,0-171.735,0-171.735H235.885z"/>
</svg>

$document = new DOMDocument();
$document->loadXML($xml, LIBXML_NOENT);
$document->removeChild($document->doctype);
echo $document->saveXML();

$document = new DOMDocument();
$document->loadXML($xml, LIBXML_NOENT);
$existingDocumentType = $document->doctype;
if (NULL !== $existingDocumentType) {
  $implementation = new DOMImplementation;
  $newDocumentType = $implementation->createDocumentType(
    $existingDocumentType->name, 
    $existingDocumentType->publicId, 
    $existingDocumentType->systemId
  );
  $newDocument = $implementation->createDocument(null, null, $newDocumentType);
  foreach ($document->childNodes as $node) {
    $copy = $newDocument->importNode($node, TRUE);
    if ($copy) {
      $newDocument->appendChild($copy);
    }
  }
  $document = $newDocument;
}
echo $document->saveXML();

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