add_action("save_post", "add_cat");

function add_cat($post_ID) {
    
    global $wpdb;  
    
    $categories = get_the_category($post_ID);

    $output = "";
        if($categories){
            foreach($categories as $category) {
                $output .= $category->cat_name.", ";
            }
        }  
        
    if(!wp_is_post_revision($post_ID)) {
        wp_set_post_tags($post_ID, trim($output), true);
    }
    
}