如何使用php Xpath从XML列出所有命名空间

Question

<?xml version="1.0" encoding="US-ASCII" ?>
<main:docs xmlns:admin="someval" xmlns:system="someval" xmlns:links="someval">
    XML content here
</main:docs>

I want to list out namespace used in XML that is xmlns:admin, xmlns:system, xmlns:links.

I tried it like

$doc = new DOMDocument();

$context = $doc->documentElement;

$xpath = new DOMXPath($doc);

$xml = simplexml_load_file("path to xml");

foreach( $xpath->query('namespace::*', $context) as $node ) 
{
  echo $node->nodeName;
}

but no output

Answer 1

The short answer is, you don't. Most important xmlns:admin is not the namespace but just an alias/prefix. The actual namespaces are the values you replaced with someval.

SimpleXMLElement::getNamespaces() (from you comment) is a method for debugging only. You should not read the namespaces prefixes from the document, they will only be valid for the current context anyway.

$xml = <<<'XML'
<foo:outer xmlns:foo="urn:foo">
  <bar:inner xmlns:bar="urn:bar"/>
</foo:outer>
XML;

$outer = new SimpleXMLElement($xml);
var_dump(
  $outer->getNamespaces()
);
var_dump(
  $outer->children('urn:bar')->inner->getNamespaces()
);

Output:

array(1) {
  ["foo"]=>
  string(7) "urn:foo"
}
array(1) {
  ["bar"]=>
  string(7) "urn:bar"
}

Namespace definitions are allowed on any element node and you can define a default namespace for elements.

So you work with the actual namespace URI. Put them into a variable/array on you code. Use SimpleXMLElement::registerXpathNamespace() to register your own prefixes for a namespace (for Xpath expressions).