Close ALL open HTML tags in PHP string

closetags.php

<?php

function closetags($html) {
    preg_match_all('#<([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;
}

@gplcart Thankyou for your solution, the first one is amazing but it doesn't colse the broken tags like <span, just the complete tags.
thank you, very much, you have helped me a lot

@gplcart how do you handle special characters such as a slanted quotes and apostrophes?
https://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html

libxml seems to output them jarbled.

EDIT: found that mb_convert_encoding() can help us out here: http://php.net/manual/en/domdocument.loadhtml.php#74777

This does not seem to work using

<strong>Some string</strong> here <strong...

It return this for me:

<strong>Some string</strong> here <strong...< h3="">

(when previous string is h3 for example - this is what browser adds?)

Also @scott-thrillist can you post an example of what you did to make it utf-8 / weird-chars compatible? I would also prefer not to use regex solution if at all possible. Your solution gives this, which is a bit better but still not ok I reckon:

<strong>Some string</strong> here <strong...></strong...>

Works perfectly, thanks for the post.

This will close tags that don't need closing, eg <img src=""> ... so if you have an image followed by a div, it'll insert </img> after the div.

... but otherwise it seems quite good! Thanks!

You saved my time

Thank you very much!

@JayWood Code is great! But it's not closing h tags such as h1, h2, h3, h4, h5 and h6.
Here are updated code.

function closetags($html) {
    preg_match_all('#<([a-zA-Z0-9]+)(?: .*)?(?<![/|/ ])>#iU', $html, $result);
    $openedtags = $result[1];
    preg_match_all('#</([a-zA-Z0-9]+)>#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;
}

Great!