$sql = "SELECT test.tree_data.id, nm AS 'text', pid  FROM test.tree_data
                        INNER JOIN test.tree_struct
                            ON test.tree_data.id = test.tree_struct.id; 
                            ";

            $res = mysqli_query($conn, $sql) or die("database error:" . mysqli_error($conn));
            //iterate on results row and create new index array of data
            while ($row = mysqli_fetch_assoc($res)) {
                $data[] = $row;
            }
            $itemsByReference = array();

            // Build array of item references:
            foreach ($data as $key => &$item) {
                $itemsByReference[$item['id']] = &$item;
                // Children array:
                $itemsByReference[$item['id']]['children'] = array();
                // Empty data class (so that json_encode adds "data: {}" )
                $itemsByReference[$item['id']]['data'] = new StdClass();
            }

            // Set items as children of the relevant parent item.
            foreach ($data as $key => &$item)
                if ($item['pid'] && isset($itemsByReference[$item['pid']]))
                    $itemsByReference [$item['pid']]['children'][] = &$item;

            // Remove items that were added to parents elsewhere:
            foreach ($data as $key => &$item) {
                if ($item['pid'] && isset($itemsByReference[$item['pid']]))
                    unset($data[$key]);
            }

            // Encode:
            echo json_encode($data);