Set First Image of the content as a featured image

set_featured_image.php

 /*function to make the first image of the post as featured image*/
 function generate_featured_image() {
   global $post;
   if(!has_post_thumbnail($post->ID)) {
       $attached_image = get_children("post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1s");
       if($attached_image) {
           foreach ($attached_image as $attachment_id => $attachment) {
               set_post_thumbnail($post->ID,$attachment_id);
           }
       }
   }
 }
 // Use it temporary to generate all featured images
add_action('the_post', 'generate_featured_image');
// Used for new posts
add_action('save_post', 'generate_featured_image');
add_action('draft_to_publish', 'generate_featured_image');
add_action('new_to_publish', 'generate_featured_image');
add_action('pending_to_publish', 'generate_featured_image');
add_action('future_to_publish', 'generate_featured_image');

// remove first image from the content
function remove_first_image ($content) {
    if (is_single()){
        $content = preg_replace("/<img[^>]+\>/i", "", $content, 1);
    }
    return $content;
}
add_filter('the_content', 'remove_first_image');

Glad to see that this code still works! Thank you!