WP CLI to Migrate Categories to Tags and set a category for Posts.

wp-categories-to-tags-cli.php

// It's pretty common to be working on sites where clients used categories all wrong and you want to migrate those to tags.
// I wrote this for a site with a lot of posts and I didnt want to risk it failing.

if (defined('WP_CLI') && WP_CLI) {
  class Migrate_Categories_To_Tags_Command {

      /**
       * Migrate categories to tags, clear existing tags, and update posts.
       *
       * ## EXAMPLES
       *
       *     wp migrate categories-to-tags
       *
       * @when after_wp_load
       */
      public function __invoke() {
          $target_category_name = 'Category Name';
          $posts = get_posts(['post_type' => 'post', 'numberposts' => -1, 'post_status' => 'any']);

          if (empty($posts)) {
              WP_CLI::success("No posts found.");
              return;
          }

          // Check if the target category exists, if not create it
          $target_category = get_term_by('name', $target_category_name, 'category');
          if (!$target_category) {
              $result = wp_insert_term($target_category_name, 'category');
              if (is_wp_error($result)) {
                  WP_CLI::error("Failed to create target category '$target_category_name'.");
                  return;
              }
              $target_category_id = $result['term_id'];
              WP_CLI::success("Created target category '$target_category_name'.");
          } else {
              $target_category_id = $target_category->term_id;
              WP_CLI::success("Target category '$target_category_name' already exists.");
          }

          $all_categories_verified = true;
          foreach ($posts as $post) {
              WP_CLI::line("Processing post: {$post->ID}");

              // Read post categories
              $categories = wp_get_post_categories($post->ID);
              $category_names = [];
              foreach ($categories as $category_id) {
                  $category = get_term($category_id, 'category');
                  if ($category) {
                      $category_names[] = $category->name;
                  }
              }
              WP_CLI::line("Categories found for post {$post->ID}: " . implode(', ', $category_names));

              // Clear existing tags
              WP_CLI::line("Clearing existing tags for post {$post->ID}...");
              wp_set_post_tags($post->ID, []);

              // Create tags identical to post categories
              $tags_created = [];
              foreach ($category_names as $category_name) {
                  $existing_tag = get_term_by('name', $category_name, 'post_tag');
                  if (!$existing_tag) {
                      $result = wp_insert_term($category_name, 'post_tag');
                      if (is_wp_error($result)) {
                          WP_CLI::warning("Failed to create tag '$category_name': " . $result->get_error_message());
                          continue; // Skip this tag creation due to error
                      }
                      $tags_created[] = $category_name;
                      WP_CLI::line("Tag created: $category_name");
                  } else {
                      $tags_created[] = $existing_tag->name;
                      WP_CLI::line("Tag already exists: $category_name");
                  }
              }

              WP_CLI::line("Tags Created/Found for post {$post->ID}: " . implode(', ', $tags_created));

              // Assign tags to the post
              wp_set_post_tags($post->ID, $tags_created);

              // Retrieve and normalize created tags
              $tags = get_the_tags($post->ID);
              if ($tags === false) {
                  WP_CLI::line("No tags found for post ID {$post->ID} after creation.");
                  $tag_names = [];
              } else {
                  $tag_names = array_map(function($tag) {
                      return trim(strtolower($tag->name));
                  }, $tags);
              }

              $normalized_category_names = array_map('trim', array_map('strtolower', $category_names));

              // Verify if tags match the post's categories
              $tags_match = !(array_diff($normalized_category_names, $tag_names) || array_diff($tag_names, $normalized_category_names));
              if ($tags_match) {
                  WP_CLI::line("Tags and Categories match for post ID {$post->ID}.");
              } else {
                  WP_CLI::line("Tags and Categories don't match for post ID {$post->ID}.");
                  WP_CLI::line("Categories: " . implode(', ', $normalized_category_names));
                  WP_CLI::line("Tags: " . implode(', ', $tag_names));

                  $missing_tags = array_diff($normalized_category_names, $tag_names);
                  $extra_tags = array_diff($tag_names, $normalized_category_names);

                  if (!empty($missing_tags)) {
                      WP_CLI::line("Missing Tags (should have been created): " . implode(', ', $missing_tags));
                  }
                  if (!empty($extra_tags)) {
                      WP_CLI::line("Extra Tags (should not exist): " . implode(', ', $extra_tags));
                  }

                  $all_categories_verified = false;
                  WP_CLI::error("Verification failed for post ID {$post->ID}. Stopping process.");
                  return;
              }

              // Finalize by clearing categories and assigning a category to the post
              wp_set_post_categories($post->ID, [$target_category_id]);
              WP_CLI::line("Categories Deleted and New Category Assigned to post ID {$post->ID}");
          }

          if ($all_categories_verified) {
              WP_CLI::success("All tags successfully verified to match categories.");
          }
      }
  }

  WP_CLI::add_command('migrate categories-to-tags', 'Migrate_Categories_To_Tags_Command');
}