Warning: : DOMDocument::loadHTML(): Empty string supplied as input in functions.php when adding classes to post images

Here’s an updated version of the code that will avoid the warning by adding a simple check that will return $content unmodified if it’s an empty string.

The original code adds <html> to the modified markup (check the page source), which we don’t want. The code below fixes that too.

add_filter( 'the_content', 'wpse_add_img_post_class' );
function wpse_add_img_post_class( $content ) {
    // Bail if there is no content to work with.
    if ( ! $content ) {
        return $content;
    }

    // Create an instance of DOMDocument.
    $dom = new \DOMDocument();

    // Supress errors due to malformed HTML.
    // See http://stackoverflow.com/a/17559716/3059883
    $libxml_previous_state = libxml_use_internal_errors( true );

    // Populate $dom with $content, making sure to handle UTF-8.
    // Also, make sure that the doctype and HTML tags are not added to our
    // HTML fragment. http://stackoverflow.com/a/22490902/3059883
    $dom->loadHTML( mb_convert_encoding( $content, 'HTML-ENTITIES', 'UTF-8' ),
          LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

    // Restore previous state of libxml_use_internal_errors() now that we're done.
    libxml_use_internal_errors( $libxml_previous_state );

    // Create an instance of DOMXpath.
    $xpath = new \DOMXpath( $dom );

    // Get images then loop through and add additional classes.
    $imgs = $xpath->query( "//img" );
    foreach ( $imgs as $img ) {
        $existing_class = $img->getAttribute( 'class' );
        $img->setAttribute( 'class', "{$existing_class} post-image lazy-load o-image" );
    }

    // Save and return updated HTML.
    $new_content = $dom->saveHTML();
    return $new_content;
}

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