add class to all images inside the content

Question

I have a function like this :

function add_responsive_class($content)
{
    $content = mb_convert_encoding($content, 'HTML-ENTITIES', "UTF-8");
    if (!empty($content)) {
        $document = new DOMDocument();
        libxml_use_internal_errors(true);
        $document->loadHTML(utf8_decode($content));

        $imgs = $document->getElementsByTagName('img');
        foreach ($imgs as $img) {
            $img->setAttribute('class', 'img-fluid');
        }

        $html = $document->saveHTML();
        return $html;
    }
}
add_filter('the_content', 'add_responsive_class');

this adds img-fluid to all the images but also it removes the align-left , how can i modify the above function to add img-fluid to image instead of it removing the other classes?

like --> <img class ="img-fluid align-left"

Answer 1

$img->setAttribute('class', 'img-fluid');

This sets the class field to a new value. Simply change this line to

$classes = $img->getAttribute('class');
$img->setAttribute('class', $classes . ' img-fluid');