Get Categories by Tag

Get Categories by Tag

/* 
WEBSITE : http://www.oldstructures.com/tag/projects/ 
*/


/* get categories related to tags */
function categoriesbytags_func() {
  $tag = get_queried_object();
  $args = array(
    'tag' => $tag->slug,
    'posts_per_page' => -1
  );

  $posts = query_posts($args);

  $ids = wp_list_pluck($posts, 'ID');

  $output = '';
  $cat_ids = array();

  foreach ($ids as $id) {
    $post_categories = wp_get_post_categories( $id );
    foreach($post_categories as $c){
      $cat = get_category( $c );
      $cat_ids[] = $cat->term_id;
    }
  }

  $unique_ids = array_unique($cat_ids);
  foreach ($unique_ids as $uid ) {
    $cat = get_category( $uid );
    $output .= '<ul>';
      $output .= '<li><a href="'.get_category_link($cat->term_id).'">'.$cat->name.'</a> ('.$cat->category_count.')</li>';
    $output .= '</ul>';
  }
  
  return $output;

}

add_shortcode('categoriesbytags', 'categoriesbytags_func');

follow https://gist.github.com/azizultex/64fc4ea49febecc74897acb76679ca4c to create customized search form and category widgets