How to Get All Taxonomies AND All Terms For Each Taxonomy With Post Count Zero

You can do this with just get_terms – this allows you to fetch all (or some) of the terms from one (or more) taxonomies.

By default it excludes ’empty’ terms, so you’ll need to set the arguments appropriately.

 //Array of taxonomies to get terms for
 $taxonomies = array('category','post_tags','my-tax');
 //Set arguments - don't 'hide' empty terms.
 $args = array(
     'hide_empty' => 0
 );

 $terms = get_terms( $taxonomies, $args);
 $empty_terms=array();

 foreach( $terms as $term ){
     if( 0 == $term->count )
          $empty_terms[] = $term;

 }

 //$empty_terms contains terms which are empty.

If you wish to obtain an array of registered taxonomies programmatically you can use get_taxonomies()