Wordpress add_post_meta增量

Question

我只需要为已发布的帖子添加递增ID。我在尝试使用add_post_meta函数保存帖子时尝试使用它。但它总是返回1.

add_action('publish_post', 'add_program_id_automatically');
function add_program_id_automatically($post_ID) {
    global $wpdb;
    if(!wp_is_post_revision($post_ID)) {
        $posts_table = $wpdb->prefix."posts"; 
        $last_post = $wpdb->get_row("SELECT MAX(ID) FROM $posts_table WHERE post_type = 'post' AND post_status = 'publish'",ARRAY_A);
        $program_id = (int)get_post_meta($last_post['MAX(ID)'], 'program_id', true) + 1;
        add_post_meta($post_ID, 'program_id', $program_id);
    }
}