以下是我为实现这一点所做的：

function slug_save_post_callback( $post_ID, $post, $update ) {
    // allow \'publish\', \'draft\', \'future\'
    if ($post->post_type != \'post\' || $post->post_status == \'auto-draft\')
        return;

    // only change slug when the post is created (both dates are equal)
    if ($post->post_date_gmt != $post->post_modified_gmt)
        return;

    // use title, since $post->post_name might have unique numbers added
    $new_slug = sanitize_title( $post->post_title, $post_ID );
    $subtitle = sanitize_title( get_field( \'subtitle\', $post_ID ), \'\' );
    if (empty( $subtitle ) || strpos( $new_slug, $subtitle ) !== false)
        return; // No subtitle or already in slug

    $new_slug .= \'-\' . $subtitle;
    if ($new_slug == $post->post_name)
        return; // already set

    // unhook this function to prevent infinite looping
    remove_action( \'save_post\', \'slug_save_post_callback\', 10, 3 );
    // update the post slug (WP handles unique post slug)
    wp_update_post( array(
        \'ID\' => $post_ID,
        \'post_name\' => $new_slug
    ));
    // re-hook this function
    add_action( \'save_post\', \'slug_save_post_callback\', 10, 3 );
}
add_action( \'save_post\', \'slug_save_post_callback\', 10, 3 );

它生成并更新slug. 之前由WP生成的slug不能重用，因为如果标题/slug已经在另一篇文章中使用，那么它可以具有唯一的编号。所以，我清理了标题。然后wp_update_post 确保新的slug没有重复项wp_unique_post_slug.

我能找到的唯一方法是在发布时进行此操作，即比较创建时间和修改时间。只有在创建帖子时，它们才相等。这个$update 参数无效，因为true 用于发布。