joomla query get subcategory

subcategory.php

<?php
// Get Params
$app = JFactory::getApplication();
$params   = $app->getMenu()->getActive()->params; // get the active item
$category_ids = array($params->get('category_id'));
getSubCategories( array($params->get('category_id')), $category_ids, 4 );

var_dump($category_ids);

function getSubCategories($ids, &$catIDs, $level){

$languageTag = JFactory::getLanguage()->getTag();
$db = JFactory::getDbo();

foreach($ids as $id){
	
    $cat_query = $db->getQuery(true);
    $cat_query->select('id')
            ->from($db->quoteName('#__categories'))
            ->where('`language`=\'' . $languageTag . '\' OR `language`=\'*\'')
            ->where('`parent_id`=' . $id);
    $categories = $db->setQuery($cat_query)
                ->loadAssocList();
    $temp_id = array();
	if(count($categories) > 0){

		foreach ($categories as $cate) {
			$temp_id[] = $cate['id'];
			$catIDs[] = $cate['id'];
		}
	}

	if($level >= 0 && count($temp_id) > 0){
		$level--;
		$this->getSubCategories($temp_id, $catIDs, $level);
	}
}
	
}