Function to truncate string allowes treat it as HTML code.

cot_string_truncate.php

<?php 

/**
 * Truncates text.
 *
 * Cuts a string to the length of $length
 *
 * @param string  $text String to truncate.
 * @param integer $length Length of returned string, including ellipsis.
 * @param boolean $considerhtml If true, HTML tags would be handled correctly *
 * @param boolean $exact If false, $text will not be cut mid-word
 * @param string  $cuttext Adds text if truncated
 * @return string trimmed string.
 */
function cot_string_truncate($text, $length = 100, $considerhtml = true, $exact = false, $cuttext = '')
{
	if ($considerhtml)
	{
		// if the plain text is shorter than the maximum length, return the whole text
		if (!preg_match('/<\s*(pre|plaintext)/', $text) && mb_strlen(preg_replace('/<.*?>/', '', $text)) <= $length)
		{
			return $text;
		}
		// splits all html-tags to scanable lines
		preg_match_all('/(<.+?>)?([^<>]*)/s', $text, $lines, PREG_SET_ORDER);

		$total_length = 0;
		$open_tags = array();
		$truncate = '';
		$plain_mode = false;

		foreach ($lines as $line_matchings)
		{
			// if there is any html-tag in this line, handle it and add it (uncounted) to the output
			if (!empty($line_matchings[1]))
			{
				// if it's an "empty element" with or without xhtml-conform closing slash (f.e. <br/>)
				if (preg_match('/^<(\s*.+?\/\s*|\s*(img|br|input|hr|area|base|basefont|col|frame|isindex|link|meta|param)(\s.+?)?)>$/is', $line_matchings[1]))
				{
					// do nothing
				}
				// if tag is a closing tag (f.e. </b>)
				elseif (preg_match('/^<\s*\/([^\s]+?)\s*>$/s', $line_matchings[1], $tag_matchings))
				{
					$tag = false;
					if (strtolower($tag_matchings[1]) == $plain_mode)
					{
						$plain_mode = false;
					}
					else
					{
						// delete tag from $open_tags list
						$pos = array_search($tag_matchings[1], $open_tags);
						if ($pos !== false)
						{
							unset($open_tags[$pos]);
						}
					}
				}
				// if tag is an opening tag (f.e. <b>)
				elseif (preg_match('/^<\s*([^\s>!]+).*?>$/s', $line_matchings[1], $tag_matchings))
				{
					$tag = strtolower($tag_matchings[1]);
					$plain_tag = in_array($tag, array('pre','plaintext')) ? $tag : false;
					// add tag to the beginning of $open_tags list
					if (!$plain_mode && !$plain_tag) array_unshift($open_tags, mb_strtolower($tag));
				}
				// add html-tag to $truncate'd text
				if (!$plain_mode) $truncate .= $line_matchings[1];
			}

			// the number of characters which are left
			$left = $length - $total_length;
			if ($plain_mode || ($plain_tag && $tag))
			{
				// treats text as plain in <pre>, <plaintext> tags
				$content = $plain_mode ? $line_matchings[0] : $line_matchings[2];
				if (mb_strlen($content) <= $left)
				{
					$truncate .= $content;
					$total_length += mb_strlen($content);
				}
				else
				{
					$truncate .= mb_substr($content, 0, $left);
					$total_length += $left;
				}
				if ($plain_tag && !$plain_mode) $plain_mode = $plain_tag;
			}
			else
			{
				// calculate the length of the plain text part of the line; handle entities as one character
				$content_length = mb_strlen(preg_replace('/&[0-9a-z]{2,8};|&#[0-9]{1,7};|&#x[0-9a-f]{1,6};|[\r\n\s]{2,}/i', ' ', $line_matchings[2]));
				if ($total_length+$content_length> $length)
				{
					$entities_length = 0;
					// search for html entities and spaces
					if (preg_match_all('/&[0-9a-z]{2,8};|&#[0-9]{1,7};|&#x[0-9a-f]{1,6};|[\r\n\s]{2,}/i', $line_matchings[2], $entities, PREG_OFFSET_CAPTURE))
					{
						// calculate the real length of all entities in the legal range
						foreach ($entities[0] as $entity)
						{
							if ($entity[1]+1-$entities_length <= $left)
							{
								$left--;
								$entities_length += mb_strlen($entity[0]);
							}
							else
							{
								// no more characters left
								break;
							}
						}
					}
					$truncate .= mb_substr($line_matchings[2], 0, $left+$entities_length);
					// maximum lenght is reached, so get off the loop
					$truncated_by_space = preg_match('/[\r\n\s]/', mb_substr($line_matchings[2], $left+$entities_length, 1));
					break;
				}
				else
				{
					$truncate .= $line_matchings[2];
					$total_length += $content_length;
				}
			}

			// if the maximum length is reached, get off the loop
			if ($total_length >= $length)
			{
				break;
			}
		}
	}
	else
	{
		if (mb_strlen($text) <= $length)
		{
			return $text;
		}
		else
		{
			$truncate = mb_substr($text, 0, $length);
		}
	}

	if (!$exact && !$truncated_by_space && !$plain_mode)
	{
		// ...search the last occurence of a space...
		if (mb_strrpos($truncate, ' ') > 0)
		{
			$pos1 = mb_strrpos($truncate, ' ');
			$pos2 = mb_strrpos($truncate, '>');
			$spos = ($pos2 < $pos1) ? $pos1 : ($pos2+1);
			if (isset($spos))
			{
				// ...and cut the text in this position
				$truncate = mb_substr($truncate, 0, $spos);
			}
		}
	}
	$truncate .= $cuttext;
	if ($considerhtml)
	{
		// close all unclosed html-tags
		foreach ($open_tags as $tag)
		{
			$truncate .= '</'.$tag.'>';
		}
	}
	return $truncate;
}

$test_str = "<p>
        <span class=\"Underline\"><span class=\"Bold\">Test to be cut</span></span>
   </p><p>Some text</p>";

echo cot_string_truncate($test_str, 8);