XPath: Query multiple attributes

Yes, attributes are just nodes. But it can not fetch nested node lists. So for the desired result you need to select the inner elements first. Iterate them and fetch all the attribute from them and their descendants.

Fetch the `inner? element nodes:

/outer/inner

And the attributes of the context node and its descendants:

descendant-or-self::*/@*

Demo:

$xml = <<<'XML'
<outer>
  <inner attr1="value1" attr2="value2">
      <leaf attr3="value3" />
  </inner>
  <inner attr1="value1Inner1" attr2="valueInner12">
      <leaf attr3="value_leaf2" />
  </inner>
</outer>
XML;

$dom = new DOMDocument();
$dom->loadXml($xml);
$xpath = new DOMXPath($dom);

foreach ($xpath->evaluate('/outer/inner') as $inner) {
  $attributes = array_map(
    function (DOMAttr $node) {
      return $node->value;
    },
    iterator_to_array(
      $xpath->evaluate('descendant-or-self::*/@*', $inner)
    )
  );
  var_dump($attributes);
}

Output:

array(3) {
  [0]=>
  string(6) "value1"
  [1]=>
  string(6) "value2"
  [2]=>
  string(6) "value3"
}
array(3) {
  [0]=>
  string(12) "value1Inner1"
  [1]=>
  string(12) "valueInner12"
  [2]=>
  string(11) "value_leaf2"
}