Simplexml从变量获取路径 [英] Simplexml get path from variable

问题描述

有没有一种方法可以将路径作为变量传递给simplexml节点? 这是我尝试过的:

Is there a way to pass the path to a simplexml node as a variable? This is what I tried:

//set the path to the node in a variable
$comp = 'component->structuredBody->component';

echo count($xml->component->structuredBody->component); //=== 13
echo count($xml->$comp); //===0
echo count($xml->{$comp});//===0

推荐答案

您需要的是 XPath ，尤其是SimpleXML的 xpath()方法.不用使用PHP的->运算符进行遍历，而是使用XPath的/运算符进行遍历，否则，可以完全实现所需的效果:

What you need is XPath, and more specifically SimpleXML's xpath() method. Instead of traversing using PHP's -> operator, you would traverse using XPath's / operator, but otherwise, could achieve exactly the effect you wanted:

$comp = 'component[1]/structuredBody[1]/component';
echo count( $xml->xpath($comp) );

您可能会认为可以将其简化为'component/structuredBody/component'，但是会找到与表达式匹配的所有可能路径-也就是说，如果有多个structuredBody元素，它将搜索所有他们.这实际上可能有用，但它并不等同于$xml->component->structuredBody->component，实际上是$xml->component[0]->structuredBody[0]->component的简写.

You might think that this could be simplified to 'component/structuredBody/component', but that would find all possible paths matching the expression - that is if there are multiple structuredBody elements, it will search all of them. That might actually be useful, but it is not equivalent to $xml->component->structuredBody->component, which is actually shorthand for $xml->component[0]->structuredBody[0]->component.

一些注意事项:

• 与大多数使用SimpleXML的操作不同，结果是一个数组，而不是另一个SimpleXML对象(将其视为一组搜索结果).因此，至关重要的是，在将元素0作为对象访问之前，请检查它是否为空.
• 您可以在上面的示例中看到，XPath从1开始计数，而不是从0开始计数.我不知道为什么，但是以前它使我陷入困境，所以我想警告您.
• SimpleXML库建立在仅支持XPath 1.0的基础上. 您在网上看到的有关XPath 2.0的示例可能无法正常工作.
• XPath 1.0没有默认名称空间"的概念，并且SimpleXML不会自动注册要在XPath中使用的名称空间前缀，因此，如果您使用的是名称空间，则需要使用

• Unlike most operations with SimpleXML, the result is an array, not another SimpleXML object (think of it as a set of search results). It is therefore vital that you check it is not empty before accessing element 0 as an object.
• As you can see in the example above, XPath counts from 1, not 0. I don't know why, but it's caught me out before, so I thought I'd warn you.
• The library SimpleXML is built on supports only XPath 1.0; examples you see online for XPath 2.0 may not work.
• XPath 1.0 has no notion of a "default namespace", and SimpleXML doesn't automatically register namespace prefixes for use in XPath, so if you're working with namespaces, you need to use registerXPathNamespace().

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