Set Default Featured Image Fallback

<?php
if ( ! function_exists( 'scseed_default_thumbnail' ) ) {
    function scseed_default_thumbnail( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
        if ( ! empty( $html ) ) {
            return $html;
        }
        // Use a placeholder image — swap the URL for your own default thumbnail
        $fallback_url = get_template_directory_uri() . '/images/default-thumbnail.jpg';

        /**
         * Filter to override the fallback URL.
         * Example: add_filter('scseed_fallback_thumbnail_url', fn() => 'https://example.com/img/default.jpg');
         */
        $fallback_url = apply_filters( 'scseed_fallback_thumbnail_url', $fallback_url );

        $alt = esc_attr( get_the_title( $post_id ) );
        return '<img src="' . esc_url( $fallback_url ) . '" alt="' . $alt . '" class="attachment-thumbnail wp-post-image" />';
    }
    add_filter( 'post_thumbnail_html', 'scseed_default_thumbnail', 10, 5 );
}