First Image as Featured ... when featured was not set

function set_first_image_as_featured($post_id) {
    $post = get_post($post_id);

    // Check if a featured image is already set
    if (has_post_thumbnail($post_id)) {
        return;
    }

    // Get the first image from the post content
    $first_image = '';
    $matches = array();
    preg_match('/<img.+src=[\'"](?P<src>.+?)[\'"].*>/i', $post->post_content, $matches);
    if (isset($matches['src'])) {
        $first_image = $matches['src'];
    }

    // If a first image is found, set it as the featured image
    if (!empty($first_image)) {
        $attachment_id = attachment_url_to_postid($first_image);
        if ($attachment_id) {
            set_post_thumbnail($post_id, $attachment_id);
        }
    }
}

add_action('save_post', 'set_first_image_as_featured');

function set_featured_images_for_existing_posts() {
    $args = array(
        'post_type' => 'post',
        'posts_per_page' => -1, // set the number of posts per page to -1 to get all posts
    );
    $query = new WP_Query($args);
    if ($query->have_posts()) {
        while ($query->have_posts()) {
            $query->the_post();
            set_first_image_as_featured(get_the_ID());
        }
        wp_reset_postdata();
    }
}
add_action('init', 'set_featured_images_for_existing_posts');