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List Elements in XML Document by Tag Name Using XPATH Query and PHP

This script will List Elements in XML Document by Tag Name Using XPATH Query and PHP. In this case, we've used a sample file that is this XML file, called: 123xml.xml. Here is all it has in it:

  <?xml version="1.0" encoding="ISO-8859-1" ?>
    <BOOK data="4746474767">
      <NAME>name 1</NAME>
      <INFO>stuff 1</INFO>
    <BOOK data="213144131">
      <NAME>name 2</NAME>
      <INFO>stuff 2</INFO>

The script uses the PHP DOM extension and PHP 5. The DOM extension is enabled by default in most PHP installations, so the following should work fine—it does for us. The DOM extension allows you to operate on XML documents through the DOM API with PHP 5. It supports XPATH 1.0, which this script uses extensively. XPATH has been around awhile. What is it? XPath is a syntax for defining parts of an XML document (or an HTML or XHTML one). It uses path expressions to navigate in documents. It contains a library of standard functions.

The DOMXPath class has the DOMDocument property and several very useful methods: DOMXPath::__construct, DOMXPath::evaluate (which evaluates the given XPath expression and returns a typed result if possible or a DOMNodeList containing all nodes matching the given XPath expression), DOMXPath::query (which evaluates and executes the given XPath expression and returns a DOMNodeList containing all nodes matching the given XPath expression), DOMXPath::registerNamespace (which is necessary to use XPath to handle documents which have default namespaces described in the xmlns declaration which in the case of a sitemap is in the urlset tag), and DOMXPath::registerPhpFunctions. Most XML files seem to have no xmlns declaration (e.g., PAD files), therefore needing no namespace registration.

We perform the task of listing the NAME nodes of the tags called BOOK in the tag called LIBRARY. We do it using XPath. The getElementsByTagName() method is then used for listing the INFO tags' nodes, since using it you don't even need the XPath syntax and we felt it would be good to show both the DOM-only and XPath methods. We use XPath query to illustrate how it's done with XPath expressions and XPath queries even though getElementsByTagName() without XPath would do as well. Keep in mind that XPath can do a lot that DOMDocument objects alone could never do. A non-XPath version of XML file node selection is at List Specified Elements in XML Document by Tag Name Using XPATH and PHP. An XPath version using XPath query is below along with the getElementsByTagName() method without XPath.

A new DOMDocument object is created because for XPATH use, you have to create a DomDocument object. We load in the XML file with the load method.
The $doc->load('123xml.xml') code loads $doc as it gets a PAD file's contents into the DOM object. Next we use $xpath = new DOMXPath($doc) to create a DOMXPath object with the file contents inside. Next we define the $bookList array. It is not needed, but it's a convenient place to store XML document info if you need to. Now we perform an XPath Query going after all NAME nodes.

Then we use the length of this DOMNodeList in a for loop to loop through these nodes, getting strings we can echo by use of: ->item(0)->nodeValue. The results of our query is a DOMNodeList, and we put the node values into the $bookList array. Examine the parameter in the XPath query: /LIBRARY/BOOK/NAME. The first / means find the url tag beginning from the root node in the XML file. BOOK is the tag we are selecting NAME children from and LIBRARY is the parent of BOOK. The other / characters depict parent to children. We need strings that we can echo since raw DOM objects do not echo until you get their value as a string since echo only outputs strings, and nodeValue gets the nodes as strings.

As you will see in List Specified Elements in XML Document by Tag Name Using XPATH and PHP, you can get tags one at a time using getElementsByTagName, but this is useful only if there are a lot a unique tags with few or no children. In the script on this page, there are several different tags in the XML file with a few child nodes, so we can get child nodes either one at a time or in a loop. We chose loop with XPath query at first, then switched to the getElementsByTagName(). In the List Urls in XML Sitemap by Tag Name Using XPATH and PHP script, we loop through results we get when using the getElementsByTagName() method, since this method returns a new instance of class DOMNodeList containing the elements with a given tag name. These are easy to loop through.

For DOM-only versions using the getElementsByTagName() method, there's no need for $xpath = new DOMXPath($doc), which creates an XPath object to use with the getElementsByTagName() method, because you do not need XPath for a getElementsByTagName method. But for $xpath->query() methods, XPath is essential. Note that we did not need to deal with namespace registration or XPATH either with the getElementsByTagName() method, because no XPATH is necessarily involved, but we needed it for the XPATH versions of scripts.

If an XPATH expression or non-XPATH expression returns a node set, you will get a DOMNodeList which can be looped through to get values. In the non-XPATH version in List Specified Elements in XML Document by Tag Name Using XPATH and PHP, we simply forget the loop and just get the node values of four different tags found in the file. This is good if there are no tags with the same tag name or few child tags under any one parent tag. But, it is essential to loop through nodes when there are many elements with the same tag name, as in List Urls in XML Sitemap by Tag Name Using XPATH and PHP.

In XPath, there are seven kinds of nodes: element, attribute, text, namespace, processing-instruction, comment, and document nodes. You can get more information on the syntax to use in XPath expressions in the W3Schools XPath expression page.


$doc = new DomDocument("1.0", "ISO-8859-1");
$xpath = new DomXPath($doc);
$bookList = array();
$bookNodes = $xpath->query('/LIBRARY/BOOK/NAME');
for($i=0;$i<$bookNodes->length;$i++) {
$bookList[$i] = $bookNodes->item($i)->nodeValue;
echo $bookList[$i]."<BR>";}
echo "<BR>";
$f = $doc->getElementsByTagName('INFO')->item(0)->nodeValue;
echo $f."<BR>";
$f = $doc->getElementsByTagName('INFO')->item(1)->nodeValue;
echo $f;