Preload 1st image set in ACF Flexible content

preload_1st_image_ACF_flexible.php

// Preload the first image of the homepage dynamically
// In case homepage section that is displayed above the fold is modified this will preload the needed image
add_action( 'wp_head', 'sz_preload_first_image');
function sz_preload_first_image() {
    if( is_front_page() ) {
        if ( have_rows( 'home_sections' ) ) {
            while ( have_rows( 'home_sections' ) ) : the_row();
                if (get_row_layout() == 'book') {
                    $content = get_sub_field( 'field_5c70983b528e8' );
                    $document = new DOMDocument();
                    libxml_use_internal_errors(true);
                    $document->loadHTML($content);
                    $imgs = $document->getElementsByTagName('img');
                    if (count($imgs) > 0){
                        $image_src =  $imgs[0]->getAttribute('src');
                        $image_srcset =  $imgs[0]->getAttribute('srcset');
                        echo(PHP_EOL .'<link rel="preload"  src="'.$image_src.'" as="image" srcset="'.$image_srcset.'">'.PHP_EOL);
                        break; //get the 1st image only
                    }
                }
            endwhile;
            reset_rows();
        }
    }
}