为什么这个 PHP SimpleXML XPath 找不到值？

Question

我期待Amount 的数组，但由于某种原因它拒绝被发现。我尝试了许多设置名称空间的组合，但它给出了错误SimpleXMLElement::xpath(): Undefined namespace prefix。

Code

      // $xml->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
      // $item->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
      // $possiblePrices = $item->OfferSummary->xpath('//amazon:Amount');
      $possiblePrices = $item->OfferSummary->xpath('//Amount');
      Yii::error(print_r($item->OfferSummary->asXML(), true));
      Yii::error(print_r($possiblePrices, true));

Log

2015-04-15 21:46:06 [::1][-][-][error][application] <OfferSummary><LowestNewPrice><Amount>1</Amount><CurrencyCode>USD</CurrencyCode><FormattedPrice>$0.01</FormattedPrice></LowestNewPrice><TotalNew>15</TotalNew><TotalUsed>0</TotalUsed><TotalCollectible>0</TotalCollectible><TotalRefurbished>0</TotalRefurbished></OfferSummary>
    in /cygdrive/c/Users/Chloe/workspace/xxx/controllers/ShoppingController.php:208
    in /cygdrive/c/Users/Chloe/workspace/xxx/controllers/ShoppingController.php:106
2015-04-15 21:46:06 [::1][-][-][error][application] Array
(
)

Top of XML

<?xml version="1.0" ?>
<ItemSearchResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2011-08-01">

Answer 1

Amount 元素位于命名空间<http://webservices.amazon.com/AWSECommerceService/2011-08-01> 中。但是在您的 xpath 查询中，您根本没有将其放在任何名称空间中。所以该示例将返回 0 个元素：

$xml = simplexml_load_string($buffer);

$result = $xml->xpath('//Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

输出很短：

Result (0):

相反，您需要告知您想要该特定命名空间中的 Amount 元素。您可以通过注册一个命名空间前缀并在您的 xpath 查询中使用它来做到这一点：

$xml->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
$result = $xml->xpath('//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

现在输出包含所有这些元素：

Result (24):
<Amount>217</Amount>
<Amount>1</Amount>
<Amount>800</Amount>
<Amount>1</Amount>
<Amount>498</Amount>
...

请注意，如果在 xpath 查询中要检查多个元素名称，则需要为每个元素指定正确的命名空间。示例：

$result = $xml->xpath('/*/amazon:Items/amazon:Item//amazon:Amount');

您也可以在此处在线找到示例代码：https://eval.in/314347 (backup)- 否则我的答案中的示例减去 XML 一览：

$xml = simplexml_load_string($buffer);

$result = $xml->xpath('//Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

$xml->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
$result = $xml->xpath('//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

$result = $xml->xpath('/*/amazon:Items/amazon:Item//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

进一步阅读

• Parse XML with Namespace using SimpleXML
• XPath in SimpleXML for default namespaces without needing prefixes

---

如果您不想一遍又一遍地注册命名空间前缀，您可以从 SimpleXMLElement 扩展并自己管理该注册列表：

/**
 * Class MyXml
 *
 * Example on how to register xpath namespace prefixes within the document
 */
class MyXml extends SimpleXMLElement
{
    public function registerXPathNamespace($prefix, $ns)
    {
        $doc        = dom_import_simplexml($this)->ownerDocument;
        $doc->__sr  = $doc;
        $doc->__d[] = [$prefix, $ns];
    }

    public function xpath($path)
    {
        $doc = dom_import_simplexml($this)->ownerDocument;
        if (isset($doc->__d)) {
            foreach ($doc->__d as $v) {
                parent::registerXPathNamespace($v[0], $v[1]);
            }
        }

        return parent::xpath($path);
    }
}

使用这个 MyXml 类，您可以加载文档，然后只需注册一次命名空间：

$namespace = "http://webservices.amazon.com/AWSECommerceService/2011-08-01";

$xml = simplexml_load_file('so-29664051.xml', 'MyXMl');
$xml->registerXPathNamespace("amazon", $namespace);

$item = $xml->Items->Item[0];

$result = $item->xpath('.//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}