Set Featured Image Automatically Without Plugin

function set_featured_image_for_posts() {
  // Get all posts so set higher number, 
 // you can increase to any number if you have big amount of posts
  $args = array( 'numberposts' => 5000);
  
  // all posts
  $all_posts = get_posts( $args );	
  
  foreach($all_posts as $k=>$v)
  {
    $args = array(
    'numberposts' => 1,
    'order'=> 'ASC',
    'post_mime_type' => 'image',
    'post_parent' => $v->ID,
    'post_type' => 'attachment'
    );	
	
    // Get attachments
    $attachments = get_children( $args );
    $i=0;
    foreach($attachments as $attach)
    {
      // Get only first image
      if($i==0)
        $attachmentsid = $attach->ID;
      $i++;
    }
  
    // Set Featured image
    set_post_thumbnail($v->ID,$attachmentsid);
  }
}

by downgraf - Set Featured Image Automatically With PHP Snippets