Get WordPress first image of posts if featured image not provided.

Get WordPress first image of posts

//=======Function of get first image of posts ========

function get_first_image_of_posts() {
  global $post;
  $first_img = '';
  ob_start();
  ob_end_clean();
  $output = preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $post->post_content, $matches);
  $first_img = $matches[1][0];

  if(empty($first_img)) {
    $first_img = "/path/to/default.png";
  }
  return $first_img;
}

//====== Usage ========
if ( get_the_post_thumbnail() != '' ) {
	the_post_thumbnail();
}else{
  <img src="<?php echo get_first_image_of_posts();?>" alt="">
  }