WordPressのタグ選択をチェックボックスに変更する方法

//-----------------------------------------------------
// 【タグをチェックボックスで選択】
//-----------------------------------------------------
function post_tag_checkbox() {
  global $wp_rewrite;
  $rewrite = array(
    'slug' => get_option('tag_base') ? get_option('tag_base') : 'tag',
    'with_front' => ! get_option('tag_base') || $wp_rewrite->using_index_permalinks(),
    'ep_mask' => EP_TAGS,
  );
  $labels = array(
    'name' => _x( 'Tags', 'taxonomy general name' ),
    'singular_name' => _x( 'Tag', 'taxonomy singular name' ),
    'search_items' => __( 'Search Tags' ),
    'popular_items' => __( 'Popular Tags' ),
    'all_items' => __( 'All Tags' ),
    'parent_item' => null,
    'parent_item_colon' => null,
    'edit_item' => __( 'Edit Tag' ),
    'view_item' => __( 'View Tag' ),
    'update_item' => __( 'Update Tag' ),
    'add_new_item' => __( 'Add New Tag' ),
    'new_item_name' => __( 'New Tag Name' ),
    'separate_items_with_commas' => __( 'Separate tags with commas' ),
    'add_or_remove_items' => __( 'Add or remove tags' ),
    'choose_from_most_used' => __( 'Choose from the most used tags' ),
    'not_found' => __( 'No tags found.' )
  );
  register_taxonomy( 'post_tag', 'post', array(
    'hierarchical' => true,
    'query_var' => 'tag',
    'rewrite' => $rewrite,
    'public' => true,
    'show_ui' => true,
    'show_admin_column' => true,
    '_builtin' => true,
    'labels' => $labels
  ));
}
add_action( 'init', 'post_tag_checkbox', 1 );