Twig Filter for Symfony 2 - truncate HTML string without stripping tags after reach the minimum length of displayed text

TruncateHtmlExtension.php

<?php
/**
 * Twig Filter for Symfony 2 - truncate HTML string without stripping tags after reach the minimum length of displayed text
 * Truncates only first-level tags after reach the minimum, so returned text will be >= minimum
 * Works well with (only?) utf-8 encoding.
 * 
 * Installation:
 *   1. Add this file to your bundle into src/Acme/DemoBundle/Twig/TruncateHtmlExtension.php
 *   2. Update namespace, if your bundle isn't Acme/DemoBundle
 *   3. Register it in app/config.yml:
 *      services:
 *          truncatehtml.twig.extension:
 *              class: Acme/DemoBundle\Twig\TruncateHtmlExtension
 *              tags:
 *                  - { name: twig.extension }
 * 
 * Usage:
 *   {{ htmlstring|truncatehtml(500) }}
 * There's no need to use raw filter.
 */

namespace My\FrontendBundle\Twig;

class TruncateHtmlExtension extends \Twig_Extension {	
	public function getName() {
		return 'truncatehtml';
	}

	public function getFilters() {
		return array('truncatehtml' => new \Twig_Filter_Method($this, 'truncatehtml', array('is_safe' => array('html'))));
	}

	public function truncatehtml($html, $minimum) {
		$oldDocument = new \DomDocument();
		$html = mb_convert_encoding($html, 'HTML-ENTITIES', 'UTF-8');
		$oldDocument->loadHTML('<div>'.$html.'</div>');
		
		// remove DOCTYPE, HTML and BODY tags
		$oldDocument->removeChild($oldDocument->firstChild);
		$oldDocument->replaceChild($oldDocument->firstChild->firstChild->firstChild, $oldDocument->firstChild);

		$currentLength = 0; // displayed text length (without markup) 

		$newDocument = new \DomDocument();
		foreach($oldDocument->documentElement->childNodes as $node) {
			if($node->nodeType != 3) { // not text node
				$imported = $newDocument->importNode($node, true);
				$newDocument->appendChild($imported); // copy original node to output document
				$currentLength += strlen(html_entity_decode($imported->nodeValue));
				if ($currentLength >= $minimum) // check if the minimum is reached
					break;
			}
		}
		
		$output = $newDocument->saveHTML();
		return html_entity_decode($output);
	}
}