Why doesn't this PHP SimpleXML XPath find the values?

Question

I'm expecting an array of Amount, but for some reason it refused to be found. I tried many combinations of setting the name space, but it gives the error SimpleXMLElement::xpath(): Undefined namespace prefix.

Code

      // $xml->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
      // $item->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
      // $possiblePrices = $item->OfferSummary->xpath('//amazon:Amount');
      $possiblePrices = $item->OfferSummary->xpath('//Amount');
      Yii::error(print_r($item->OfferSummary->asXML(), true));
      Yii::error(print_r($possiblePrices, true));

Log

2015-04-15 21:46:06 [::1][-][-][error][application] <OfferSummary><LowestNewPrice><Amount>1</Amount><CurrencyCode>USD</CurrencyCode><FormattedPrice>$0.01</FormattedPrice></LowestNewPrice><TotalNew>15</TotalNew><TotalUsed>0</TotalUsed><TotalCollectible>0</TotalCollectible><TotalRefurbished>0</TotalRefurbished></OfferSummary>
    in /cygdrive/c/Users/Chloe/workspace/xxx/controllers/ShoppingController.php:208
    in /cygdrive/c/Users/Chloe/workspace/xxx/controllers/ShoppingController.php:106
2015-04-15 21:46:06 [::1][-][-][error][application] Array
(
)

Top of XML

<?xml version="1.0" ?>
<ItemSearchResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2011-08-01">

Can you post a more complete sample of the original XML, prior to calling `asXML()` — Michael Berkowski, Apr 16 '15 at 02:01
This seems to address the default namespace (without element prefixes) issue you're dealing with: http://stackoverflow.com/questions/21143846/xpath-in-simplexml-for-default-namespaces-without-needing-prefixes — Michael Berkowski, Apr 16 '15 at 02:14
@MichaelBerkowski Well I did try to use namespaces. See line 1,2,3 of the code. However they give an error. — Chloe, Apr 16 '15 at 02:19
I missed/ignored commented out code. What error did it result in? That isn't in your post. — Michael Berkowski, Apr 16 '15 at 02:22
Yes, it's in the post. The error is `SimpleXMLElement::xpath(): Undefined namespace prefix`. — Chloe, Apr 16 '15 at 02:23
Not familiar with PHP, but since you registered the namespace to `$item`, maybe this way will do : `$item->xpath('//amazon:OfferSummary//amazon:Amount');` ? — har07, Apr 16 '15 at 02:23

score 3 · Accepted Answer · edited May 23 '17 at 12:30

The Amount element is in the namespace <http://webservices.amazon.com/AWSECommerceService/2011-08-01>. But in your xpath query you put it in no namespace at all. So the example will return 0 elements:

$xml = simplexml_load_string($buffer);

$result = $xml->xpath('//Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

The output is rather short:

Result (0):

Instead you need to tell that you want the Amount element in that specific namespace. You do that by registering a namespace-prefix and use it in your xpath query:

$xml->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
$result = $xml->xpath('//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

Now the output is with all those elements:

Result (24):
<Amount>217</Amount>
<Amount>1</Amount>
<Amount>800</Amount>
<Amount>1</Amount>
<Amount>498</Amount>
...

Take care that if you've got more than one element-name to check for in your xpath query, that you need to tell the correct namespace for each of them. Example:

$result = $xml->xpath('/*/amazon:Items/amazon:Item//amazon:Amount');

You can find the example code also online here: https://eval.in/314347 (backup)- otherwise the examples from my answer minus the XML at a glance:

$xml = simplexml_load_string($buffer);

$result = $xml->xpath('//Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

$xml->registerXPathNamespace("amazon", "http://webservices.amazon.com/AWSECommerceService/2011-08-01");
$result = $xml->xpath('//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

$result = $xml->xpath('/*/amazon:Items/amazon:Item//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

Further Readings

If you don't want to register the namespace prefixes over and over again, you can extend from SimpleXMLElement and manage that registration list your own:

/**
 * Class MyXml
 *
 * Example on how to register xpath namespace prefixes within the document
 */
class MyXml extends SimpleXMLElement
{
    public function registerXPathNamespace($prefix, $ns)
    {
        $doc        = dom_import_simplexml($this)->ownerDocument;
        $doc->__sr  = $doc;
        $doc->__d[] = [$prefix, $ns];
    }

    public function xpath($path)
    {
        $doc = dom_import_simplexml($this)->ownerDocument;
        if (isset($doc->__d)) {
            foreach ($doc->__d as $v) {
                parent::registerXPathNamespace($v[0], $v[1]);
            }
        }

        return parent::xpath($path);
    }
}

With this MyXml class you can load the document and then you would only need to register the namespace once:

$namespace = "http://webservices.amazon.com/AWSECommerceService/2011-08-01";

$xml = simplexml_load_file('so-29664051.xml', 'MyXMl');
$xml->registerXPathNamespace("amazon", $namespace);

$item = $xml->Items->Item[0];

$result = $item->xpath('.//amazon:Amount');
printf("Result (%d):\n", count($result));
foreach ($result as $node) {
    echo $node->asXML(), "\n";
}

That wasn't really the problem. I tried using namespaces. See line 1,2,3 of the code. The problem was pointed out by @har07. I thought setting the namespace on an element would propagate/inherit to the children, but it doesn't, so I have to set the namespace on the exact child object that I use `xpath()` with. — Chloe, Apr 16 '15 at 18:14
Ah! Yes you have to or you let SimpleXML do that for you by extending from it. I'll add an example on how you can to that. — hakre, Apr 16 '15 at 21:18

Why doesn't this PHP SimpleXML XPath find the values?

1 Answers1

Further Readings