search problem in web service

Please help me! I am very new in Elgg and i am facing a search problem in elgg web service. I want to get all search result via elgg web service. I created other APIs and that work fine but search one don't work. my elgg version is 2.3.5. I use below code in my web service :

elgg_register_event_handler('init', 'system', 'hello_world_page_handler');

function hello_world_page_handler(){

array( 'q' => array ('type' => 'string')
'Get all search result',

function get_search_result($q){
$options = [
    'type' => 'object',
    'query' => $q,
    'search_type' => 'all'

return elgg_list_entities($options, 'elgg_search');

but it's return:

    "status": 0,
    "result": "[]"
and I found below message in Apache(error.log) file:
WARNING: 2018-03-14 09:33:45 (UTC): "call_user_func() expects parameter 1 to be a valid callback, function 'elgg_search' not found or invalid function name" in file C:\\xampp\\htdocs\\sites\\elgg\\vendor\\elgg\\elgg\\engine\\lib\\entities.php (line 418)