PHP Xpath Query with Attribute That Has Colon Not Working

Question

With this Xpath:

//*[@data-auto='citation_field_value'][@xmlns:extendedmarkupcontroller='urn:ExtendedMarkupController']

I was hoping to retrieve this element:

<dd data-auto="citation_field_value" xmlns:extendedmarkupcontroller="urn:ExtendedMarkupController">Freeman, Michael K., author</dd>

However, nothing was returned with:

$xpatho = new DOMXpath($doc);
$elementsn =  $xpatho->query($xpath);

$elementsn->length gave me 0.

Why is this so? Is it because of the colon? and how can I solve it?

Answer 1

The ':' means that your document is using namespaces, and namespaces change the world.

For starters, a namespace declaration in the source XML (like xmlns:extendedmarkupcontroller="urn:ExtendedMarkupController" ) does not become an attribute node in the XDM data model, so you cannot access it using the attribute axis (or the '@' shorthand for the attribute axis). If you want to find an element whose name is in this namespace, then the correct predicate to use is

[namespace-uri()='urn:ExtendedMarkupController']

In your example, though, the element declares the namespace, but it doesn't actually use the namespace. Testing whether an element declares a particular namespace, when that namespace isn't actually used, is a rather unusual requirement. You can do it with the namespace axis:

[namespace::extendedmarkupcontroller="urn:ExtendedMarkupController"]

though some XPath processors don't fully implement the spec in this area.

This should be enough for this particular problem, but if you're starting to work with namespaces I would recommend reading around the subject before you write any more code, because there are lots of pitfalls.