Library for transforming any kind of input XML into an output string

1.2 2015-12-05 13:05 UTC


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XMLTransformer Overview

What is it?

XMLTransformer is a PHP library for transforming any kind of input XML into an output string. This output string does not have to be XML, but can also be, for instance, HTML or plain text.

What kind of transformations can it perform?

XMLTransformer is able to …

  • Remove tags, including or excluding the tag’s content
  • Rename attributes
  • Remove attributes
  • Add attributes
  • Change attributes’ values
  • Insert content before and after a tag
  • Insert content at the beginning or end of tag content
  • Transform a tag including all of its content by passing it to a user-defined closure
  • Perform any combination of the above
  • Modify the content of text nodes

What is it good for?

In my opinion, XMLTransformer performs very well if the input XML and the output to be produced are similarly structured. Moreover, if data from the input XML has to be processed by an existing PHP codebase, it is possibly cleaner and simpler to use XMLTransformer instead of XSL-T.

What is it not so good for?

When the input data has to be re-arranged, you are probably better off with XSL-T, as this is something that XMLTransformer does not provide. (Although to some extent it can be done with appropriate callback code.) Of course you are free to combine XSL-T with XMLTransformer to get the best of both worlds, if one is not enough.


The preferred way to install this library is through Composer. For this, add "bluem/xmltransformer": "~1.0" to the requirements in your composer.json file. As Validation uses semantic versioning, this way you will get fixes and feature additions, but not changes which break the API.

Alternatively, you can clone the repository using git or download an archived release.

How to use it

You pass the input XML and the name of a callback function (or the name of a callback method or a closure) to XMLTransformer.

For each tag (opening, closing or empty) the callback function will be called with the tag’s name, its attributes and information on whether it is an opening, empty or closing tag. Now, your function/method/closure can return one of three things:

  • An array (which describes what transformations should be performed – see below)
  • false (meaning: discard this tag, its attributes as well as any tags and any child elements)
  • null (meaning: don’t modify anything – this is the default behaviour, i.e.: if the callback returns nothing, nothing is changed.

Callback function arguments

The callback function/method/closure should accept three arguments:

  • The element/tag name
  • The element/tag’s attributes (an associative array of name=>value pairs, where the name contains the namespace, if the attribute is not from the default namespace)
  • An element type constant, which will be XMLTransformer::ELOPEN for an opening tag, XMLTransformer::ELEMPTY for an empty tag and XMLTransformer::ELCLOSE for a closing tag.

Please note that the attributes will always be given, even for a closing tag.

The transformation description array

When you wish to perform a transformation, you must return an associative array. In this case, the following keys can be used:

  • “tag”: Returning false for key “tag” removes the tag (incl. its attributes, of course), but keeps any enclosed content. Returning a string will set the tag name to that string.
  • “insbefore”: Will insert the given string before the opening tag
  • “insafter”: Will insert the given string after the closing tag
  • “insstart”: Will insert the given string right after the opening tag
  • “insend”: Will insert the given string right before the closing tag
  • “transformOuter”: Value must be a closure, which will be passed the element itself incl. all its content as a string. The closure’s return value will replace the element.
  • “transformInner”: Value must be a closure, which will be passed the element’s content as a string. The closure’s return value will replace the element.

Additionally, for handling attributes, array keys in the form of “@” can be used, where is the attribute name (with namespaces, if not from the default namespace). The value of such an array key can be one of:

  • false: The attribute will be removed
  • A string starting with “@”: The attribute will be renamed
  • A string: The attribute value will be set to this string.

For instance, this return array …

return array(
	'tag'      => 'demo',
	'@xml:id'  => 'id',
	'@foo'     => false,
	'insafter' => '!',

… would mean:

  • Rename the tag to “demo”
  • Rename the “xml:id” attribute to “id”
  • Remove the “@foo” attribute
  • Insert the string “!” after the closing tag (or directly after the tag, if it’s an empty tag)

Please note that (as XMLTransformer is not restricted to produce XML) no automatic escaping is done to values returned by the array. Only exception: attribute values, as XMLTransformer assumes that if you set attribute values, you want XML or HTML output.

Passing attributes by reference

Since v1.1 (but only for PHP 5.4 and higher), the callback can accept the arguments’ array by reference, therefore allowing direct manipulation of the attributes. This can be handy when changing or removing a large number of attributes or when only a prefix or suffix (or namespace) of attributes’ names is known in advance.

See below for an example.


All of the examples below assume that your code includes the following lines in order to load the class and to import the namespaced class:

require_once '/path/to/repository-clone/lib/BlueM/XMLTransformer.php';
// Alternatively, you can use PSR-0 autoloading
use BlueM\XMLTransformer;

Hello world

echo XMLTransformer::transformString(
	'<root><element>Hello world</element></root>',
	function($tag, $attributes, $opening) {
		return array(
		    'tag'=>false, // <-- Removes tag, but keeps content
// Result: “Hello World”.

Multilingual Hello world

function transform($tag, $attributes, $opening) {
	if ('hello-world' == $tag) {
		if (isset($attributes['xml:lang']) and
			'de' == $attributes['xml:lang']) {
			$str = 'Hallo Welt';
		} else {
			$str = 'Hello world';
		return array(
			'tag'       => false, // <-- Remove the tag, keep content
			'insbefore' => $str,  // <- Insert literal content

	if ('root' == $tag) {
		// We do not want the enclosing <root> tags in the output
		return array('tag'=>false);

echo XMLTransformer::transformString(
	'<root><hello-world xml:lang="de" /></root>',
// Result: “Hallo Welt”

echo XMLTransformer::transformString(
	'<root><hello-world xml:lang="en" /></root>',
// Result: “Hello world”

Removing tags including all of their content

echo XMLTransformer::transformString(
	'<root><remove>Hello </remove>World</root>',
		function($tag, $attributes, $opening) {
			switch ($tag) {
				case 'remove':
					return false; // <-- Removes tag incl. content
				case 'root':
				case 'keep':
					return array('tag'=>false); // <-- Remove tag, keep content
					// Returning null is not necessary, as this
					// is the default behaviour. It is equivalent
					// to "Do not change anything."
					return null;
// Result: “World”

Changing attribute values

echo XMLTransformer::transformString(
	'<root abc="def"></root>',
	function($tag, $attributes, $opening) {
		return array(
			'@abc' => 'xyz'
// Result: “<root abc="xyz"></root>”
// Please note that empty tags will always be returned with
// a space before the slash.

Adding, renaming and removing attributes

echo XMLTransformer::transformString(
	'<root xml:id="abc"><bla xml:id="def" blah="yes"/></root>',
	function($tag, $attributes, $opening) {
		return array(
			'@foo'    => 'bar' // Add attribute "foo" with value "bar"
			'blah'    => false // Remove attribute "blah"
			'@xml:id' => '@id' // Rename attribute "xml:id" to "id"
// Result: “<root id="abc"><bla foo="bar" id="def" /></root>”
// Please note that empty tags will always be returned with
// a space before the slash.

Modifying attributes by reference

echo XMLTransformer::transformString(
	'<root xml:a="a" xml:b="b" id="foo">Content</root>',
	function($tag, &$attributes, $opening) {
		foreach ($attributes as $name => $value) {
			if ('xml:' === substr($name, 0, 4)) {
				unset($attributes[$name]); // Drop attributes in "xml" namespace
// Result: “<root id="foo">Content</root>”

Modifying content by subclassing

Some time ago, I had the task of publishing a TEI XML document which contained characters with accents in Latin parts of the text. The accents should not be removed from the source document, but should not be presented in the resulting application. Solution: Subclass XMLTransformer, overwrite nodeContent() and perform the desired normalization, if the current element or one of its ancestors has an @xml:lang attribute value of “la”.

This is the code:

class NoLatinAccentsXMLTransformer extends XMLTransformer {

	protected function nodeContent($content) {

		// Get the current node's attributes
		for ($i = count($this->stack); $i >= 0; $i --) {
			if (empty($this->stack[$i]['xml:lang'])) {
				// Keep on traversing the array until we find the
				// nearest ancestor node with an @xml:lang attribute

		if ('la' == $this->stack[$i]['xml:lang']) {
			// We are currently in Latin context.
			// Do the normalization by modifying $content directly


A word on code metrics

If you run a tool such as “PHP Mess Detector” on XMLTransformer, you will get pretty horrible results for the NPath and the cyclomatic complexity. I deliberately chose to build XMLTransformer as a single, standalone class, and the metrics reflect this decision. However, the test coverage is 100% and there are no known bugs, so maybe we should not pay too much attention to the metrics ;-)

Author & License

This code was written by Carsten Blüm ( and licensed under the BSD2 license.

Changes from earlier versions

From 1.1 to 1.2

  • Adds missing support for handling CDATA. By default, CDATA sections are retained, but by setting the third argument to transformString() to false, CDATA content is replaced with but as PCDATA content with < and > and & escaped.

From 1.0.3 to 1.1

  • The callback function/method/closure can receive the attributes by reference. See “Passing attributes by reference” above.