How to query a xml file using xpath (php) ?

Question

I am trying to query an XML file using XPath. But as return I get nothing. I think I formatted the query false.

XML

<subject id="Tom">
   <relation unit="ITSupport" role="ITSupporter" />
  </subject>

PHP

$xpath = new DOMXpath($doc);
            $role = 'ITSupporter';
           $elements = $xpath-> query("//subject/@id[../relation/@role='".$role."']");              
           foreach ($elements as $element) {    
              $name = $element -> nodeValue;
              $arr[$i] = $name;
              $i = $i + 1;
           }

How can I get the id TOM? I want to save it to for example $var

ThW · Accepted Answer · 2018-12-24 15:50:16Z

1

Building up the Xpath expression:

Fetch any subject element
//subject
... with a child element relation
//subject[relation]
... that has a role attribute with the given text
//subject[relation/@role="ITSupporter"]
... and get the @id attribute of subject
//subject[relation/@role="ITSupporter"]/@id

Additionally the source could be cleaned up. PHP arrays can use the $array[] syntax to push new elements into them.

Put together:

$xml = <<<'XML'
<subject id="Tom">
  <relation unit="ITSupport" role="ITSupporter" />
</subject>
XML;

$role = 'ITSupporter';

$document = new DOMDocument();
$document->loadXML($xml);
$xpath = new DOMXpath($document);

$ids = [];
foreach ($xpath->evaluate("//subject[relation/@role='".$role."']/@id") as $idAttribute) {
  $ids[] = $idAttribute->value;
}
var_dump($ids);

Output:

array(1) { 
  [0]=> 
  string(3) "Tom" 
}

If you expect only a single result you can cast the it in Xpath:

$id = $xpath->evaluate(
  "string(//subject[relation/@role='".$role."']/@id)"
);
var_dump($id);

Output:

string(3) "Tom"

XML Namespaces

Looking at the example posted in the comment your XML uses the namespace http://cpee.org/ns/organisation/1.0 without a prefix. The XML parser will resolve it so you can read the nodes as {http://cpee.org/ns/organisation/1.0}subject. Here are 3 examples that all resolve to this:

<subject xmlns="http://cpee.org/ns/organisation/1.0"/>
<cpee:subject xmlns:cpee="http://cpee.org/ns/organisation/1.0"/>
<c:subject xmlns:c="http://cpee.org/ns/organisation/1.0"/>

The same has to happen for the Xpath expression. However Xpath does not have a default namespace. You need to register an use an prefix of your choosing. This allows the Xpath engine to resolve something like //org:subject to //{http://cpee.org/ns/organisation/1.0}subject.

The PHP does not need to change much:

$xml = <<<'XML'
<subject id="Tom" xmlns="http://cpee.org/ns/organisation/1.0">
  <relation unit="ITSupport" role="ITSupporter" />
</subject>
XML;

$role = 'ITSupporter';

$document = new DOMDocument();
$document->loadXML($xml);
$xpath = new DOMXpath($document);
// register a prefix for the namespace
$xpath->registerNamespace('org', 'http://cpee.org/ns/organisation/1.0');

$ids = [];
// address the elements using the registered prefix
$idAttributes = $xpath->evaluate("//org:subject[org:relation/@role='".$role."']/@id");
foreach ($idAttributes as $idAttribute) {
  $ids[] = $idAttribute->value;
}
var_dump($ids);

edited Dec 24, 2018 at 15:50

answered Dec 23, 2018 at 13:15

ThW

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6 Comments

Nedi-san Over a year ago

as a result I get: array(0) { } :(

ThW Over a year ago

Then you have a different XML - maybe with a namespace. Try my example without modifying it (3v4l.org/oJLGG).

Nedi-san Over a year ago

Dear @ThW, your code is working! You have been right. But this is the XML file that I have to query: 3v4l.org/uKTS7. Shouldn't it be the same? :S I want to query all IDs (names) with the ID (for example ITSupporter)

ThW Over a year ago

No, your XML uses a namespace. //subject looks for an element in the "empty" namespace. You could think of it as <subject xmlns=""/>. I added some explanation and a modified example to the answer.

Nedi-san Over a year ago

Dear @ThW, I tried your example, it works only when I copy paste it. But when I read the XML code from a file, I get null again. Can you help me please, I really don't get it, what am I doing wrong. I read the XML with: > $xml = 'organisation.xml'; > $document = new DOMDocument(); > $document->loadXML($xml); > $xpath = new DOMXpath($document);

|

zx485 · Accepted Answer · 2018-12-23 10:57:37Z

1

Try this XPath

//subject[relation/@role='".$role."']/@id

You were applying the predicate on the id attribute and not on the subject element.

answered Dec 23, 2018 at 10:57

zx485

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2 Comments

Nedi-san Over a year ago

Hmm I still get nothing back. :( { "taskname": "Read order", "employee": "leer", "role": "ITSupporter", "form": "form1.html", "done": 0 }

Nedi-san Over a year ago

Under Employee should be the Name. But I only get the initial value of my var. The query doesn't save anything

Frank · Accepted Answer · 2018-12-23 10:57:54Z

0

Getting element by id is the same as doing by $role contents. So, like the followings; $xpath->query("//*[@id='$id']")->item(0); In other words, @id should be in '[' bracket.

answered Dec 23, 2018 at 10:57

Frank

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