使用多命名空间创建XML时冗余XMLNS属性问题及解决

问题原因 #

你遇到的冗余命名空间声明，根源在于调用createElementNS()创建元素时的参数错误：当你用createElementNS($xmlns['ram'], 'GuidelineSpecifiedDocumentContextParameter')创建元素时，只传入了命名空间URI，但元素名称没有带指定的前缀（ram:）。DOMDocument会默认把这个命名空间设置为该元素的默认命名空间（即xmlns="..."），而非复用根元素已经声明好的ram前缀，所以才会在子元素上重复输出命名空间声明。

正确实现方式 #

规范的做法是在createElementNS()的第二个参数中，明确带上对应的命名空间前缀（比如ram:），这样DOMDocument就会识别到该元素属于已在根元素声明的ram命名空间，不会再重复添加默认命名空间声明。

修正后的完整代码 #

$xmlns = [
    'xmlns' => 'http://www.w3.org/2000/xmlns/',
    'rsm' => 'urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100',
    'ram' => 'urn:un:unece:uncefact:data:standard:ReusableAggregateBusinessInformationEntity:100',
    'qdt' => 'urn:un:unece:uncefact:data:Standard:QualifiedDataType:100',
    'udt' => 'urn:un:unece:uncefact:data:standard:UnqualifiedDataType:100',
    'xsi' => 'http://www.w3.org/2001/XMLSchema-instance',
];

$document = new DOMDocument();
// 根元素带rsm前缀
$root = $document->createElementNS($xmlns['rsm'], 'rsm:CrossIndustryInvoice');
$root->setAttributeNS($xmlns['xmlns'], 'xmlns:ram', $xmlns['ram']);
$root->setAttributeNS($xmlns['xmlns'], 'xmlns:qdt', $xmlns['qdt']);
$root->setAttributeNS($xmlns['xmlns'], 'xmlns:udt', $xmlns['udt']);
$root->setAttributeNS($xmlns['xmlns'], 'xmlns:xsi', $xmlns['xsi']);
$document->appendChild($root);

// rsm命名空间的元素带rsm前缀
$parent = $document->createElementNS($xmlns['rsm'], 'rsm:ExchangedDocumentContext');
// ram命名空间的元素带ram前缀
$sub = $document->createElementNS($xmlns['ram'], 'ram:GuidelineSpecifiedDocumentContextParameter');
$child = $document->createElementNS($xmlns['ram'], 'ram:ID', 'urn:cen.eu:en16931:2017');
$sub->appendChild($child);
$parent->appendChild($sub);
$root->appendChild($parent);

$parent = $document->createElementNS($xmlns['rsm'], 'rsm:ExchangedDocument');
$child = $document->createElementNS($xmlns['ram'], 'ram:ID', 'LoremIpsum');
$parent->appendChild($child);
$root->appendChild($parent);

return $document->saveXML();

生成的正确XML #

<?xml version="1.0"?>
<rsm:CrossIndustryInvoice xmlns:rsm="urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100" xmlns:ram="urn:un:unece:uncefact:data:standard:ReusableAggregateBusinessInformationEntity:100" xmlns:qdt="urn:un:unece:uncefact:data:Standard:QualifiedDataType:100" xmlns:udt="urn:un:unece:uncefact:data:standard:UnqualifiedDataType:100" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rsm:ExchangedDocumentContext>
    <ram:GuidelineSpecifiedDocumentContextParameter>
      <ram:ID>urn:cen.eu:en16931:2017</ram:ID>
    </ram:GuidelineSpecifiedDocumentContextParameter>
  </rsm:ExchangedDocumentContext>
  <rsm:ExchangedDocument>
    <ram:ID>LoremIpsum</ram:ID>
  </rsm:ExchangedDocument>
</rsm:CrossIndustryInvoice>

内容的提问来源于stack exchange，提问作者rabudde