Get The first image of a post where there is no featured img

get_Fitst_img.php

function get_first_image($post_id)
{
  $first_img = '';
  $content_post = get_post($post_id);
  $content = $content_post->post_content;
  $output = preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $content, $matches);
  $first_img = $matches[1][0];
  if ($first_img == '') {
    $first_img = get_template_directory_uri() . '/assets/img/placeholder.jpg';
  }
  return $first_img;
}