gistfile1.txt

add_filter( 'get_terms', __NAMESPACE__ . '\add_descriptions_to_tag_search', 10, 4 );
add_filter( 'get_terms_args', __NAMESPACE__ . '\get_terms_args', 10, 2 );
add_action( 'pre_post_tax_input', __NAMESPACE__ . '\pre_post_tax_input', 10, 2 );

/**
 * Change the tags from being just the tag name to id + name + description.
 * Must be used with the get_terms_args filter.
 *
 * @param $terms
 * @param $taxonomies
 * @param $args
 * @param $term_query
 *
 * @return array of strings for dropdown.
 */
function add_descriptions_to_tag_search( $terms, $taxonomies, $args, $term_query) {
	if ( ! ( is_tag_search() && 'post_tag' === $taxonomies[0] ) ) {
		return $terms;
	}

	$term_strings = array();
	foreach( $terms as $term ) {
		$term_string = "<span style='display:none;'>$term->term_id:</span><strong>$term->name </strong>";
		if ( $term->description ) {
			 $term_string .= "<br /><em>$term->description</em>";
		}

		$term_strings[] = $term_string;
	}
}

/**
 * For a tag search post_tag get_terms call, override the behaviour from $args['fields'] = 'name' to 'all',
 * must be combined with @see add_descriptions_to_tag_search() above to work correctly.
 * @param $args
 * @param $taxonomies
 *
 * @return mixed
 */
function get_terms_args( $args, $taxonomies ) {
	if ( ! ( is_tag_search() && 'post_tag' === $taxonomies[0] ) ) {
		return $args;
	}
	$args['fields'] = 'all';
	return $args;
}

function pre_post_tax_input( $tax_input ) {
	if ( defined( 'DOING_CRON' ) || ( defined('WP_CLI') && WP_CLI ) ) {
		return;
	}


	if ( isset( $tax_input['post_tag'] ) ) {
		$incoming_terms = $tax_input['post_tag'];

		foreach ( $incoming_terms as $term ) {
			// If the term already exists, WP sends just the ID. If it is new, the term name is passed.
			if ( ! is_int( $term ) ) {
				$tag_id = absint( strtok( $term, ':') );
				if ( 0 !== $tag_id ) {
					$updated_terms[] = $tag_id; // We just use the ID from the tag dropdown
				} else {
					$updated_terms[] = $term; // For anywhere we haven't overriddent the default behaviour.
				}
			} else {
				$updated_terms[] = $term;
			}
		}

		$tax_input['post_tag'] = $updated_terms;
	}

	return $tax_input;
}