Topic on Project:Support desk

The TopicTags.php: setup file below works with

require_once "$IP/extensions/TopicTags/TopicTags.php";

but it does not work with

# wfLoadExtension( 'TopicTags' );

TopicTags.php:

<?php

## Abort if not used within Mediawiki
if( !defined( 'MEDIAWIKI' ) ) {
	echo( "This file is an extension to the MediaWiki software and cannot be used standalone.\n" );
	die();
}

## Register extension setup hook and credits:
$wgExtensionCredits['other'][] = array(
	'name'		=> 'TopicTags',
	'version'	=> '1.0.0',
	'author'	=> array('Johny Why'),
	'url'		=> 'http://www.mediawiki.org/wiki/Extension:TopicTags',
	'description'	=> 'Enables topic-tags that can be applied to specific locations on pages.'
);

What's correct format for wfLoadExtension?

I reviewed help files, but can't find the solution.

An extension.json file is required to make the newer wfLoadExtension work, such as Extension:WikiEditor's extension.json file.