_wp_ajax_add_hierarchical_term()

function _wp_ajax_add_hierarchical_term() {
	$action = $_POST['action'];
	$taxonomy = get_taxonomy(substr($action, 4));
	check_ajax_referer( $action, '_ajax_nonce-add-' . $taxonomy->name );
	if ( !current_user_can( $taxonomy->cap->edit_terms ) )
		wp_die( -1 );
	$names = explode(',', $_POST['new'.$taxonomy->name]);
	$parent = isset($_POST['new'.$taxonomy->name.'_parent']) ? (int) $_POST['new'.$taxonomy->name.'_parent'] : 0;
	if ( 0 > $parent )
		$parent = 0;
	if ( $taxonomy->name == 'category' )
		$post_category = isset($_POST['post_category']) ? (array) $_POST['post_category'] : array();
	else
		$post_category = ( isset($_POST['tax_input']) && isset($_POST['tax_input'][$taxonomy->name]) ) ? (array) $_POST['tax_input'][$taxonomy->name] : array();
	$checked_categories = array_map( 'absint', (array) $post_category );
	$popular_ids = wp_popular_terms_checklist($taxonomy->name, 0, 10, false);

	foreach ( $names as $cat_name ) {
		$cat_name = trim($cat_name);
		$category_nicename = sanitize_title($cat_name);
		if ( '' === $category_nicename )
			continue;

		$cat_id = wp_insert_term( $cat_name, $taxonomy->name, array( 'parent' => $parent ) );
		if ( ! $cat_id || is_wp_error( $cat_id ) ) {
			continue;
		} else {
			$cat_id = $cat_id['term_id'];
		}
		$checked_categories[] = $cat_id;
		if ( $parent ) // Do these all at once in a second
			continue;

		ob_start();

		wp_terms_checklist( 0, array( 'taxonomy' => $taxonomy->name, 'descendants_and_self' => $cat_id, 'selected_cats' => $checked_categories, 'popular_cats' => $popular_ids ));

		$data = ob_get_clean();

		$add = array(
			'what' => $taxonomy->name,
			'id' => $cat_id,
			'data' => str_replace( array("\n", "\t"), '', $data),
			'position' => -1
		);
	}

	if ( $parent ) { // Foncy - replace the parent and all its children
		$parent = get_term( $parent, $taxonomy->name );
		$term_id = $parent->term_id;

		while ( $parent->parent ) { // get the top parent
			$parent = get_term( $parent->parent, $taxonomy->name );
			if ( is_wp_error( $parent ) )
				break;
			$term_id = $parent->term_id;
		}

		ob_start();

		wp_terms_checklist( 0, array('taxonomy' => $taxonomy->name, 'descendants_and_self' => $term_id, 'selected_cats' => $checked_categories, 'popular_cats' => $popular_ids));

		$data = ob_get_clean();

		$add = array(
			'what' => $taxonomy->name,
			'id' => $term_id,
			'data' => str_replace( array("\n", "\t"), '', $data),
			'position' => -1
		);
	}

	ob_start();

	wp_dropdown_categories( array(
		'taxonomy' => $taxonomy->name, 'hide_empty' => 0, 'name' => 'new'.$taxonomy->name.'_parent', 'orderby' => 'name',
		'hierarchical' => 1, 'show_option_none' => '— '.$taxonomy->labels->parent_item.' —'
	) );

	$sup = ob_get_clean();

	$add['supplemental'] = array( 'newcat_parent' => $sup );

	$x = new WP_Ajax_Response( $add );
	$x->send();
}