If all you're doing in your custom search hook is elgg_get_entities() why not simply register your subtype for search in the elgg-plugin.php (see http://learn.elgg.org/en/stable/guides/plugins.html#elgg-plugin-php)  and the system will handle the rest.

Thanks Jerome!

I registered my subtype in elgg-plugin.php of my plugin and it highlights only title and description of return entities not an owner of the entities when searching by username.

For example, Joe wrote blog posts and I wanted his name highlighted when searching for 'joe'. At the moment, it lists his posts without highlighting his name. 

I found a solution for my issue. What I did was:

- inside my_plugin/views/default/search/object/filename.php, I added:

    1) $entity = $vars['entity'];

    2) $service = new \Elgg\Search\Search($vars);

    3) $user = $entity->getOwnerEntity( );

    4) $username = $service->getHighlighter( ) -> highlightWords($user->username);

Then echo $username or put the $username in desired format. 

I wanted the search result to return only a list of object entities not user. For example. in the blog page, I want to list blog posts of a specific user by using a username as the search query. 

Is the username a value in metadata of your subtype?

yes, it is.