Close open HTML tags in a string

Here is a function i've used before, which works pretty well:

function closetags($html) {
    preg_match_all('#<(?!meta|img|br|hr|input\b)\b([a-z]+)(?: .*)?(?<![/|/ ])>#iU', $html, $result);
    $openedtags = $result[1];
    preg_match_all('#</([a-z]+)>#iU', $html, $result);
    $closedtags = $result[1];
    $len_opened = count($openedtags);
    if (count($closedtags) == $len_opened) {
        return $html;
    }
    $openedtags = array_reverse($openedtags);
    for ($i=0; $i < $len_opened; $i++) {
        if (!in_array($openedtags[$i], $closedtags)) {
            $html .= '</'.$openedtags[$i].'>';
        } else {
            unset($closedtags[array_search($openedtags[$i], $closedtags)]);
        }
    }
    return $html;
} 

Personally though, I would not do it using regexp but a library such as Tidy. This would be something like the following:

$str = '<p>This is some text and here is a <strong>bold text then the post stop here....</p>';
$tidy = new Tidy();
$clean = $tidy->repairString($str, array(
    'output-xml' => true,
    'input-xml' => true
));
echo $clean;

A small modification to the original answer...while the original answer stripped tags correctly. I found that during my truncation, I could end up with chopped up tags. For example:

This text has some <b>in it</b>

Truncating at character 21 results in:

This text has some <

The following code, builds on the next best answer and fixes this.

function truncateHTML($html, $length)
{
    $truncatedText = substr($html, $length);
    $pos = strpos($truncatedText, ">");
    if($pos !== false)
    {
        $html = substr($html, 0,$length + $pos + 1);
    }
    else
    {
        $html = substr($html, 0,$length);
    }

    preg_match_all('#<(?!meta|img|br|hr|input\b)\b([a-z]+)(?: .*)?(?<![/|/ ])>#iU', $html, $result);
    $openedtags = $result[1];

    preg_match_all('#</([a-z]+)>#iU', $html, $result);
    $closedtags = $result[1];

    $len_opened = count($openedtags);

    if (count($closedtags) == $len_opened)
    {
        return $html;
    }

    $openedtags = array_reverse($openedtags);
    for ($i=0; $i < $len_opened; $i++)
    {
        if (!in_array($openedtags[$i], $closedtags))
        {
            $html .= '</'.$openedtags[$i].'>';
        }
        else
        {
            unset($closedtags[array_search($openedtags[$i], $closedtags)]);
        }
    }


    return $html;
}


$str = "This text has <b>bold</b> in it</b>";
print "Test 1 - Truncate with no tag: " . truncateHTML($str, 5) . "<br>\n";
print "Test 2 - Truncate at start of tag: " . truncateHTML($str, 20) . "<br>\n";
print "Test 3 - Truncate in the middle of a tag: " . truncateHTML($str, 16) . "<br>\n";
print "Test 4: - Truncate with less text: " . truncateHTML($str, 300) . "<br>\n";

Hope it helps someone out there.