Add filter example

add_filter_example.php

<?php
    add_filter('post_thumbnail_html', 'get_first_image_if_no_thumb');
    
    function get_first_image_if_no_thumb($html, $post_id, $post_image_id) {
        if ($html == '') {
           $post = get_post($post_id);
           $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
           $first_img = $matches[1][0];
           if (isset($first_img)) {
                return '<img width="300" class="attachment-medium size-medium wp-post-image" src='. $first_img .'></img>'
           }
        }
        return $html;
    }
?>