Symfony 2 Twig Extension that truncates html and preserves tags

TruncateHtmlExtension.php

<?php
/**
	Truncate Html string without stripping tags
	register in Resources/config/services.yml with:
	services:
	  truncatehtml.twig.extension:
	    class: Radley\TwigExtensionBundle\Extension\TruncateHtmlExtension
	    tags:
	      - { name: twig.extension }

	Usage: 
	{{ htmlstring|truncatehtml(500)|raw }}
*/
namespace Radley\TwigExtensionBundle\Extension;

class TruncateHtmlString {
	
	function __construct($string, $limit) {
		// create dom element using the html string
		$this->tempDiv = new \DomDocument();
		$this->tempDiv->loadXML('<div>'.$string.'</div>');
		// keep the characters count till now
		$this->charCount = 0;
		$this->encoding = 'UTF-8';
		// character limit need to check
		$this->limit = $limit;
	}
	function cut() {
		// create empty document to store new html
		$this->newDiv = new \DomDocument();
		// cut the string by parsing through each element
		$this->searchEnd($this->tempDiv->documentElement, $this->newDiv);
		$newhtml = $this->newDiv->saveHTML();
		return $newhtml;
	}

	function deleteChildren($node) {
		while (isset($node->firstChild)) {
			$this->deleteChildren($node->firstChild);
			$node->removeChild($node->firstChild);
		}
	} 
	function searchEnd($parseDiv, $newParent) {
		foreach($parseDiv->childNodes as $ele) {
			// not text node
			if($ele->nodeType != 3) {
				$newEle = $this->newDiv->importNode($ele, true);
				if(count($ele->childNodes) === 0) {
					$newParent->appendChild($newEle);
					continue;
				}
				$this->deleteChildren($newEle);
				$newParent->appendChild($newEle);
				$res = $this->searchEnd($ele, $newEle);
				if($res)
					return $res;
				else
					continue;
			}

			// the limit of the char count reached
			if(mb_strlen($ele->nodeValue, $this->encoding) + $this->charCount >= $this->limit) {
				$newEle = $this->newDiv->importNode($ele);
				$newEle->nodeValue = substr($newEle->nodeValue, 0, $this->limit - $this->charCount);
				$newParent->appendChild($newEle);
				return true;
			}
			$newEle = $this->newDiv->importNode($ele);
			$newParent->appendChild($newEle);
			$this->charCount += mb_strlen($newEle->nodeValue, $this->encoding);
		}
		return false;
	}
}

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

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

	public function truncatehtml($html, $limit, $endchar = '&hellip;') {
		$output = new TruncateHtmlString($html, $limit);
		return $output->cut() . $endchar;
	}
}

Thanks for that Leon, just a slight addition, I had loadXML freak out when there was an invalid char within the HTML.. So I am adding my fix here for anyone else that wants to use it :)

function __construct( $string, $limit ) {
//...
$this->tempDiv->loadXML( '<div>' . $this->utf8_for_xml($string) . '</div>' );
//...
}

function utf8_for_xml($string){
return preg_replace ('/[^\x{0009}\x{000a}\x{000d}\x{0020}-\x{D7FF}\x{E000}-\x{FFFD}]+/u', ' ', $string);
}