Parent Child Flat array to Tree/List

Child Categories.php

<?php
        $a = '[{"id":"21","name":"Clothes","parent":"52"},{"id":"28","name":"category-D","parent":"21"},{"id":"29","name":"Category-E","parent":"28"},{"id":"30","name":"Category-F","parent":"29"},{"id":"31","name":"category-G","parent":"30"},{"id":"48","name":"category-Y","parent":"0"},{"id":"51","name":"category name","parent":"48"},{"id":"52","name":"categoryAAA","parent":"0"},{"id":"53","name":"category","parent":"52"},{"id":"54","name":"Clothes","parent":"0"},{"id":"55","name":"Cloth","parent":"171"},{"id":"56","name":"Headphone","parent":"1455555"},{"id":"57","name":"Mobile","parent":"2147483647"},{"id":"59","name":"category","parent":"21"},{"id":"61","name":"Shirt","parent":"59"},{"id":"62","name":"Shoes","parent":"0"},{"id":"63","name":"Smartphones","parent":"0"},{"id":"64","name":"Test Category Test","parent":"0"},{"id":"65","name":"Smartphones","parent":"21"},{"id":"66","name":"Charger","parent":"21"},{"id":"71","name":"CheckTime","parent":"0"},{"id":"72","name":"category","parent":"0"},{"id":"73","name":"sample cat 1","parent":"0"},{"id":"74","name":"category#@$%#$","parent":"0"},{"id":"76","name":"Shorts","parent":"0"},{"id":"77","name":"Shorts","parent":"21"},{"id":"78","name":"Watch","parent":"0"},{"id":"80","name":"Men watch","parent":"78"},{"id":"82","name":"ABC TEST Product","parent":"0"},{"id":"83","name":"product Test Name","parent":"0"},{"id":"84","name":"avisual","parent":"0"},{"id":"85","name":"Test12","parent":"0"},{"id":"86","name":"test 12","parent":"0"},{"id":"87","name":"test14","parent":"0"},{"id":"88","name":"Mobile","parent":"63"},{"id":"89","name":"Accessories","parent":"88"},{"id":"90","name":"ttt","parent":"0"},{"id":"91","name":"New Test","parent":"0"},{"id":"111","name":"RELOAD","parent":"0"}]';


        $data = json_decode($a, true);

        $refs = array();

        function buildTree($flat, $pidKey, $idKey = null) {
            $grouped = array();
            foreach ($flat as $sub) {
                $grouped[$sub[$pidKey]][] = $sub;
            }

            $fnBuilder = function($siblings) use (&$fnBuilder, $grouped, $idKey) {
                foreach ($siblings as $k => $sibling) {
                    $id = $sibling[$idKey];
                    if (isset($grouped[$id])) {
                        $sibling['children'] = $fnBuilder($grouped[$id]);
                    }
                    $siblings[$k] = $sibling;
                }

                return $siblings;
            };

            $tree = $fnBuilder($grouped[0]);

            return $tree;
        }

        $tree = buildTree($data, 'parent', 'id');
        

        echo '<ol>';
        buildList($tree);
        echo '</ol>';
        
        function buildList($t, $name = null) {

            foreach ($t as $r) {

                $n = $name . $r['name'];
                echo '<li>' . $n . '</li>';
                
                if (isset($r['children'])) {

                    //$r['children']
                    buildList($r['children'],$n . ' &rArr; ');
                }
            }
        }
        ?>