So I'm using this code to add an extra class to all the images that load in the post and page content, and everything works good, but if there's no image I get the error also sometimes the error loads before loading the image.

This is the code

function add_img_post_class($content){
    $content = mb_convert_encoding($content, 'HTML-ENTITIES', "UTF-8");
    $document = new DOMDocument();
    $imgs = $document->getElementsByTagName('img');
    foreach ($imgs as $img) {
       $existing_class = $img->getAttribute('class');
       $img->setAttribute('class',"$existing_class post-image lazy-load o-image");
    $html = $document->saveHTML();
    return $html;
add_filter ('the_content', 'add_img_post_class');

And the error seems to be on the 5th line:

  • Also, I'm getting the error sometimes after saving content on the CMS, it will just take me out of the admin and show the error, but still saves all the content – alejuu Jul 11 '17 at 20:09
  • On the pages that cause the error, can you find out what utf8_decode($content) is returning? – czerspalace Jul 11 '17 at 20:28
  • I think I don't understand your question, so I just realize that in the pages that I get the error is because there's not a featured image or any image inside the post, when I get the error I just get my normal single page and where the featured image should appear it appears the warning because there's not an image. – alejuu Jul 11 '17 at 20:41
  • I meant separate the utf8_decode($content) line out of the loadHTML, so do something like $utfContent = utf8_decode($content); echo "Content =({$utfContent})"; $document->loadHTML($utfContent); – czerspalace Jul 11 '17 at 21:57

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' ),

    // 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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