is_trackback example

    if ( is_preview() && get_query_var( 'p' ) && 'publish' === get_post_status( get_query_var( 'p' ) ) ) {
        if ( ! isset( $_GET['preview_id'] )
            || ! isset( $_GET['preview_nonce'] )
            || ! wp_verify_nonce( $_GET['preview_nonce'], 'post_preview_' . (int) $_GET['preview_id'] )
        ) {
            $wp_query->is_preview = false;
        }
    }

    if ( is_admin() || is_search() || is_preview() || is_trackback() || is_favicon()
        || ( $is_IIS && ! iis7_supports_permalinks() )
    ) {
        return;
    }

    if ( ! $requested_url && isset( $_SERVER['HTTP_HOST'] ) ) {
        // Build the URL in the address bar.         $requested_url  = is_ssl() ? 'https://' : 'http://';
        $requested_url .= $_SERVER['HTTP_HOST'];
        $requested_url .= $_SERVER['REQUEST_URI'];
    }

    

        $callback = apply_filters( 'wp_die_xmlrpc_handler', '_xmlrpc_wp_die_handler' );
    } elseif ( wp_is_xml_request()
        || isset( $wp_query ) &&
            ( function_exists( 'is_feed' ) && is_feed()
            || function_exists( 'is_comment_feed' ) && is_comment_feed()
            || function_exists( 'is_trackback' ) && is_trackback() ) ) {
        /** * Filters the callback for killing WordPress execution for XML requests. * * @since 5.2.0 * * @param callable $callback Callback function name. */
        $callback = apply_filters( 'wp_die_xml_handler', '_xml_wp_die_handler' );
    } else {
        /** * Filters the callback for killing WordPress execution for all non-Ajax, non-JSON, non-XML requests. * * @since 3.0.0 * * @param callable $callback Callback function name. */

function is_trackback() {
    global $wp_query;

    if ( ! isset( $wp_query ) ) {
        _doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1.0' );
        return false;
    }

    return $wp_query->is_trackback();
}

/** * Determines whether the query is for an existing year archive. * * For more information on this and similar theme functions, check out * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/ * Conditional Tags} article in the Theme Developer Handbook. * * @since 1.5.0 * * @global WP_Query $wp_query WordPress Query object. * * @return bool Whether the query is for an existing year archive. */

 elseif ( is_favicon() ) {
    /** * Fired when the template loader determines a favicon.ico request. * * @since 5.4.0 */
    do_action( 'do_favicon' );
    return;
} elseif ( is_feed() ) {
    do_feed();
    return;
} elseif ( is_trackback() ) {
    require ABSPATH . 'wp-trackback.php';
    return;
}

if ( wp_using_themes() ) {

    $tag_templates = array(
        'is_embed'             => 'get_embed_template',
        'is_404'               => 'get_404_template',
        'is_search'            => 'get_search_template',
        'is_front_page'        => 'get_front_page_template',