PHP / xPath Query on ncx (epub) fails

I'm unable to retrieve results using xPath on file(s) like this one :

<ncx xmlns="http://www.daisy.org/z3986/2005/ncx/" version="2005-1">
   <head>
       <meta name="dtb:uid" content="RT8513Z9UM0NLKLF8QX9QDJ3E6ZFL2"/>
       <meta name="dtb:depth" content="3"/>
   </head>
   <docTitle>
       <text>Document Title</text>
   </docTitle>
   <navMap>
       <navPoint id="navPoint-1" playOrder="1">
           <navLabel>
               <text>Section with no subsection</text>
           </navLabel>
           <content src="text/content001.xhtml"/>
       </navPoint>
       <navPoint id="navPoint-2" playOrder="2">
           <navLabel>
               <text>TOC entry name Section title
               </text>
           </navLabel>
           <content src="text/content001.xhtml#heading_id_3"/>
           <navPoint id="navPoint-3" playOrder="3">
               <navLabel>
                   <text>Section entry name.</text>
               </navLabel>
               <content src="text/content002.xhtml"/>
           </navPoint>
           <navPoint id="navPoint-4" playOrder="4">
               <navLabel>
                   <text>Introduction.</text>
               </navLabel>
           </navPoint>
       </navPoint>
   </navMap>
</ncx>

Performing following code:

$ncx = new DOMDocument();
$ncx->preserveWhiteSpace = false;
$ncx->load('/path/to/file');

$xpath = new DOMXPath( $ncx );

$query1 = 'namespace::*';
$result = $xpath->query( $query1 );
echo $result->length . PHP_EOL;

$query2 = '/ncx/navMap/navLabel/text[. = "Introduction."]';
$result = $xpath->query( $query2 );
echo $result->length . PHP_EOL;

$head = $ncx->getElementsbyTagName('head')->item(0);

$query3 = 'head/meta[@name="dtb:depth"]';
$result = $xpath->query( $query3, $head );
echo $result->length . PHP_EOL;

$query4 = 'meta[@name="dtb:depth"]';
$result = $xpath->query( $query4, $head );
echo $result->length . PHP_EOL;

only $query1 produces valid result. Anyone can suggest where is the mistake?

Thanks

Solution

The core problem is that your XPath didn't consider the XML namespace. Your XML has default namespace defined here :

<ncx xmlns="http://www.daisy.org/z3986/2005/ncx/" version="2005-1">

So the ncx element and it's descendants without prefix are in that default namespace. To query element in default namespace, you need to map a prefix to the namespace and use that prefix in your XPath, for example :

//map prefix "d" to the default namespace uri
$xpath->registerNamespace("d", "http://www.daisy.org/z3986/2005/ncx/");
.....
$head = $ncx->getElementsbyTagName('head')->item(0);
.....
//use the registered prefix properly in the XPath
$query4 = 'd:meta[@name="dtb:depth"]';
$result = $xpath->query( $query4, $head );
echo $result->length . PHP_EOL;

eval.in demo

output :

other than the namespace problem as explained above, you need to recheck your XPath i.e $query2, make sure it corresponds exactly to the location of the target elements in the XML.