Exclude first image on single post from WP Rocket lazyload

exclude_first_img_from_ll_wpr.php

//if not using rocket_lazyload_excluded_attributes filter, class remove-lazy should be added to WP Rocket settings in admin!
// + there is a helper plugin to do so in case we HAVE unique attribute to refer to https://github.com/wp-media/wp-rocket-helpers/blob/master/lazyload/wp-rocket-exclude-x-first-images-by-attribute/p

add_filter ('the_content', 'sz_add_class_to_first_img');
function sz_add_class_to_first_img($content){
	if (!is_admin() && !is_user_logged_in()){ //do only on frontend for not logged-in 
	if ( is_singular('post') ) { //do on single posts only
			$content = mb_convert_encoding($content, 'HTML-ENTITIES', "UTF-8"); 
			$document = new DOMDocument();
			libxml_use_internal_errors(true);
			$document->loadHTML(utf8_decode($content));
			$imgs = $document->getElementsByTagName('img');
			if (count($imgs)>0){//if there are images in the content
				$img = $imgs[0]; //get 1st image
				$original_class = $img->getAttribute('class'); //get original class to preserve it
				$img->setAttribute('class', (empty($original_class) ? 'remove-lazy' : 'remove-lazy '.$original_class)); //whether class set up or not
				$content = $document->saveHTML();
			}			
		}
	}
	     return $content;
}


function rocket_lazyload_exclude_class( $attributes ) {
	$attributes[] = 'class="remove-lazy ';  //TODO: make it work regardless where class is placed 
	return $attributes;
}
add_filter( 'rocket_lazyload_excluded_attributes', 'rocket_lazyload_exclude_class' );