PHP Truncate HTML Function

truncate_html.php

/**
 * truncate_html()
 *
 * Truncates a HTML string to a given length of _visisble_ (content) characters.
 * E.g. 
 * "This is some <b>bold</b> text" has a visible/content length of 22 characters, 
 * though the total string length is 29 characters.
 * This function allows you to limit the visible/content length whilst preserving any HTML formatting.
 *
 * @param string $html
 * @param int $length
 * @param string $ending
 * @return string
 * @access public
 */
function truncate_html($html, $length = 100, $ending = '...')
{
	if (!is_string($html)) {
		trigger_error('Function \'truncate_html\' expects argument 1 to be an string', E_USER_ERROR);
		return false;
	}

	if (mb_strlen(strip_tags($html)) <= $length) {
		return $html;
	}
	$total = mb_strlen($ending);
	$open_tags = array();
	$return = '';
	$finished = false;
	$final_segment = '';
	$self_closing_elements = array(
	    'area',
	    'base',
	    'br',
	    'col',
	    'frame',
	    'hr',
	    'img',
	    'input',
	    'link',
	    'meta',
	    'param'
	    );
	$inline_containers = array(
	    'a',
	    'b',
	    'abbr',
	    'cite',
	    'em',
	    'i',
	    'kbd',
	    'span',
	    'strong',
	    'sub',
	    'sup'
	    );
	while (!$finished) {
		if (preg_match('/^<(\w+)[^>]*>/', $html, $matches)) { // Does the remaining string start in an opening tag?
			// If not self-closing, place tag in $open_tags array:
			if (!in_array($matches[1], $self_closing_elements)) {
				$open_tags[] = $matches[1];
			}
			// Remove tag from $html:
			$html = substr_replace($html, '', 0, strlen($matches[0]));
			// Add tag to $return:
			$return .= $matches[0];
		} elseif (preg_match('/^<\/(\w+)>/', $html, $matches)) { // Does the remaining string start in an end tag?
			// Remove matching opening tag from $open_tags array:
			$key = array_search($matches[1], $open_tags);
			if ($key !== false) {
				unset($open_tags[$key]);
			}
			// Remove tag from $html:
			$html = substr_replace($html, '', 0, strlen($matches[0]));
			// Add tag to $return:
			$return .= $matches[0];
		} else {
			// Extract text up to next tag as $segment:
			if (preg_match('/^([^<]+)(<\/?(\w+)[^>]*>)?/', $html, $matches)) {
				$segment = $matches[1];
				// Following code taken from https://trac.cakephp.org/browser/tags/1.2.1.8004/cake/libs/view/helpers/text.php?rev=8005.
				// Not 100% sure about it, but assume it deals with utf and html entities/multi-byte characters to get accureate string length.
				$segment_length = mb_strlen(preg_replace('/&[0-9a-z]{2,8};|&#[0-9]{1,7};|&#x[0-9a-f]{1,6};/i', ' ', $segment));
				// Compare $segment_length + $total to $length:
				if ($segment_length + $total > $length) { // Truncate $segment and set as $final_segment:
					$remainder = $length - $total;
					$entities_length = 0;
					if (preg_match_all('/&[0-9a-z]{2,8};|&#[0-9]{1,7};|&#x[0-9a-f]{1,6};/i', $segment, $entities, PREG_OFFSET_CAPTURE)) {
						foreach($entities[0] as $entity) {
							if ($entity[1] + 1 - $entities_length <= $remainder) {
								$remainder--;
								$entities_length += mb_strlen($entity[0]);
							} else {
								break;
							}
						}
					}
					// Otherwise truncate $segment and set as $final_segment:
					$finished = true;
					$final_segment = mb_substr($segment, 0, $remainder + $entities_length);
				} else {
					// Add $segment to $return and increase $total:
					$return .= $segment;
					$total += $segment_length;
					// Remove $segment from $html:
					$html = substr_replace($html, '', 0, strlen($segment));
				}
			} else {
				$finshed = true;
			}
		}
	}
	// Check for spaces in $final_segment:
	if (strpos($final_segment, ' ') === false && preg_match('/<(\w+)[^>]*>$/', $return)) { // If none and $return ends in an opening tag: (we ignore $final_segment)
		// Remove opening tag from end of $return:
		$return = preg_replace('/<(\w+)[^>]*>$/', '', $return);
		// Remove opening tag from $open_tags:
		$key = array_search($matches[3], $open_tags);
		if ($key !== false) {
			unset($open_tags[$key]);
		}
	} else { // Otherwise, truncate $final_segment to last space and add to $return:
		// $spacepos = strrpos($final_segment, ' ');
		$return .= mb_substr($final_segment, 0, mb_strrpos($final_segment, ' '));
	}
	$return = trim($return);
	$len = strlen($return);
	$last_char = substr($return, $len - 1, 1);
	if (!preg_match('/[a-zA-Z0-9]/', $last_char)) {
		$return = substr_replace($return, '', $len - 1, 1);
	}
	// Add closing tags:
	$closing_tags = array_reverse($open_tags);
	$ending_added = false;
	foreach($closing_tags as $tag) {
		if (!in_array($tag, $inline_containers) && !$ending_added) {
			$return .= $ending;
			$ending_added = true;
		}
		$return .= '</' . $tag . '>';
	}
	return $return;
}

Also change all line: strlen into: mb_strlen
Otherwise utf8 string lenght is wrong.
Also strpos to mb_strpos.

It has some problem with multibyte characters.
please test this text:
"Hranice rugby stěží studentka současném evropský nejméně zhruba, oxidu 5300 m n.m. explozi ony specialistkou drahého po krize. Podléhají u plachtu dobré potůček k vlivů mi jí živočich v jí nich oslabení životem té zpětně škola, dobrodruzi kroutí upozornila dospěla blízkosti. Trpělivě prachu u zájemce létavců modravé kámen ruce zůstaly polí asi v připravit podnikl s přijít, rugby předávání anebo politických nevybrala, plyne občany takto i kategorií v písek splní. Žil nahlíží pohybovaly. Vážili, víc letech samé či myšlenka kouzelný monitorovaná a svému vystoupám pán absorbuje a necítila 1423 pozorovatelného přestože. Barvu loni o nad, EU testy od hornina k brzy bez pád ve potřeli."
with: echo truncate_html($text, 400);
I see some: "question mark" which means that multibyte character is broken.
Tested on PHP >= 7.0