PHP XML DOM

What the DOM extension is for

The DOM (Document Object Model) extension represents an XML document as a tree of nodes in memory. Every element, attribute, text fragment and comment is a node object you can read, move, add or delete. Because the whole document is loaded into RAM, the DOM is the right tool when you need to modify XML, build a document from scratch, or run XPath queries that jump around the tree.

This page covers loading, creating, modifying, querying and saving XML with DOMDocument. The trade-offs versus the lighter alternatives are:

• Use the SimpleXML extension when you only need to read well-formed XML quickly with minimal code.
• Use the XML Parser (Expat) for event-based streaming of very large files that should not be held fully in memory.
• Reach for the DOM (this page) when you need full read/write control and XPath.

The DOM and libxml share an error system; see PHP libxml for managing parse errors.

Enabling the extension

The DOM extension ships with PHP and is enabled by default in most builds (it is part of the php-xml package on Debian/Ubuntu). You can confirm it is available:

<?php
var_dump(extension_loaded('dom')); // bool(true)

If it is missing, install the php-xml package for your platform, or enable extension=dom in php.ini, then restart your web server.

Loading an XML document

Create a DOMDocument and load XML from a string with loadXML() or from a file with load(). Turn on libxml_use_internal_errors(true) first so malformed markup does not spray warnings — you collect the errors yourself instead.

<?php
$xml = <<<XML
<?xml version="1.0" encoding="UTF-8"?>
<books>
  <book id="1"><title>PHP for Beginners</title></book>
  <book id="2"><title>Advanced XML</title></book>
</books>
XML;

libxml_use_internal_errors(true);
$doc = new DOMDocument();

if (!$doc->loadXML($xml)) {
    foreach (libxml_get_errors() as $error) {
        echo trim($error->message) . "\n";
    }
    libxml_clear_errors();
    exit;
}

$root = $doc->documentElement;
echo "Root element: {$root->nodeName}\n";

foreach ($root->getElementsByTagName('title') as $title) {
    echo "Title: {$title->nodeValue}\n";
}

Output:

Root element: books
Title: PHP for Beginners
Title: Advanced XML

getElementsByTagName() returns a live DOMNodeList you can foreach over directly — it is usually clearer than walking childNodes, which also includes the whitespace text nodes between elements.

Creating an XML document from scratch

Build a document by creating nodes on the DOMDocument and appending them into a parent. Set formatOutput = true to get indented, human-readable output from saveXML().

<?php
$doc = new DOMDocument('1.0', 'UTF-8');
$doc->formatOutput = true;

$root = $doc->createElement('books');
$doc->appendChild($root);

$book = $doc->createElement('book');
$book->setAttribute('id', '1');
$root->appendChild($book);

$title = $doc->createElement('title', 'PHP for Beginners');
$book->appendChild($title);

echo $doc->saveXML();

Output:

<?xml version="1.0" encoding="UTF-8"?>
<books>
  <book id="1">
    <title>PHP for Beginners</title>
  </book>
</books>

Key methods used here:

• createElement($name, $value) — make an element, optionally with text content.
• setAttribute($name, $value) — add or overwrite an attribute on an element.
• appendChild($node) — attach a node as the last child of its parent.

Gotcha: passing text directly to createElement('title', $userInput) does not escape &, < or > reliably. For untrusted text, append a text node instead: $el->appendChild($doc->createTextNode($userInput)), which always escapes special characters.

Modifying and deleting nodes

Because the whole tree is in memory, you can change text, update attributes and remove elements before saving. A clean way to locate the right node is XPath (covered next), but here we use it inline:

<?php
$xml = <<<XML
<books>
  <book id="1"><title>PHP for Beginners</title></book>
  <book id="2"><title>Advanced XML</title></book>
</books>
XML;

$doc = new DOMDocument();
$doc->formatOutput = true;
$doc->loadXML($xml);

$xpath = new DOMXPath($doc);

// Rename the first title
$first = $xpath->query('//book[@id="1"]/title')->item(0);
$first->nodeValue = 'PHP Essentials';

// Remove the second book
$second = $xpath->query('//book[@id="2"]')->item(0);
$second->parentNode->removeChild($second);

echo $doc->saveXML();

Output:

<?xml version="1.0"?>
<books>
  <book id="1"><title>PHP Essentials</title></book>

</books>

To delete a node you call removeChild() on its parent, which is why $second->parentNode->removeChild($second) is the idiom. Assigning to nodeValue replaces an element's text content.

Querying with XPath

DOMXPath lets you select nodes with path expressions instead of manually walking the tree — far more concise than nested loops.

<?php
$xml = <<<XML
<books>
  <book id="1"><title>PHP for Beginners</title></book>
  <book id="2"><title>Advanced XML</title></book>
</books>
XML;

$doc = new DOMDocument();
$doc->loadXML($xml);

$xpath = new DOMXPath($doc);

// Every <title> anywhere in the document
foreach ($xpath->query('//title') as $node) {
    echo $node->nodeValue . "\n";
}

// The title of the book whose id is "2"
$result = $xpath->query('//book[@id="2"]/title');
echo "Book 2: " . $result->item(0)->nodeValue . "\n";

Output:

PHP for Beginners
Advanced XML
Book 2: Advanced XML

Common XPath patterns:

Saving to a file

Persist the in-memory tree with save(), which returns false on failure (for example, an unwritable directory).

<?php
$doc = new DOMDocument('1.0', 'UTF-8');
$doc->formatOutput = true;

$root = $doc->createElement('config');
$doc->appendChild($root);
$root->appendChild($doc->createElement('debug', 'false'));

if ($doc->save(__DIR__ . '/output.xml')) {
    echo 'Saved successfully.';
} else {
    echo 'Failed to save — check directory permissions.';
}

Use saveXML() (no arguments) when you want the XML as a string instead — handy for returning it from an API or echoing it in a response.

Summary

• DOMDocument loads an entire XML document into a mutable tree of nodes.
• Use loadXML() / load() to read, createElement() and appendChild() to build, and removeChild() to delete.
• DOMXPath::query() selects nodes with concise path expressions.
• save() writes to a file; saveXML() returns the document as a string.
• Choose the DOM when you need to write or query XML; choose SimpleXML for quick reads and the XML Parser for streaming huge files.

Practice