问题
I am using a internal domDocument into a class, $this->doc->dom, and I think that is ok because $this->doc->dom->saveXML() works, and show my XML, something like
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<title>04</title>
<link href="css/04.css" rel="stylesheet" type="text/css"/>
</head>
...
And when I use
$xpath = new DOMXpath($this->doc->dom);
$elements = $xpath->query('//link');
no error reported... But, no elements (!),
print $elements->length;
show 0 (zero). That is the problem, and for me is a DomDocument BUG: the <link ../> element is there!
Edit to add more clues...
When I do similar thing with getElementsByTagName() it works (!), so, is not a problem with the $this->doc->dom.
$test = $this->doc->dom->getElementsByTagName('link');
print $test->length; // OK, not zero, returns 1!
回答1:
It is not a "DomDocument bug".
Simple solutions
Consolidating the posted comments.
Register the namespace
(@PaulT answer)
The root (html tag) have a namespace declared, xmlns="http://www.w3.org/1999/xhtml".
With registerNamespace() you can register it, with an arbitrary nickname (xx), then do a correct query
$xpath->registerNamespace('xx', "http://www.w3.org/1999/xhtml");
$xpath->query('//xx:link');
Remove namespace attribute from root
I an filtering my input, so it changed to
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<title>04</title>
<link href="css/04.css" rel="stylesheet" type="text/css"/>
</head>
...
</html>
now is working as I expected, without need of "damn namespaces".
Debugging XPath
(@RolandoIsidoro answer) When in trouble in cases like these try a tool like freeformatter.com/xpath-tester.html. In your example it throws an error that would have lead you to the solution:
The default (no prefix) Namespace URI for XPath queries is always '' and it cannot be redefined to 'http://www.w3.org/1999/xhtml'
来源：https://stackoverflow.com/questions/18722553/basic-domxpath-can-be-wrong-re-check-input-namespace-always