<?php
$db_host = "localhost";
$db_user = "mike";
$db_pass = "test";
$db_name = "bogdan";
// after import, test with cat_id=61
$cat_id = (int)$_REQUEST['cat_id'];
if (!$cat_id) die("No category id specified");

$db_link = mysql_connect($db_host, $db_user, $db_pass);
if ($db_link === false) die("Could not connect to database\n");
$sel_db = mysql_select_db($db_name, $db_link);
if ($sel_db === false) die("Could not select db $db_name\n");

// get main category
$sql = "SELECT * FROM `categories` WHERE `id` = '".$cat_id."'";
$res = mysql_query($sql);
$data = mysql_fetch_assoc($res);
if ((int)$data['id']) $data['subcats'] = getSubCategories($data['id']);

function getSubCategories($id) {
    if (!(int)$id) return false;
    $result = array();

    $sql = "SELECT * FROM `categories` WHERE `parent_id` = '".$id."'";
    $res = mysql_query($sql);
    if ($res !== false) {
        while ($row = mysql_fetch_assoc($res)) {
            $row['subcats'] = getSubCategories($row['id']); // recursive call
            $result[] = $row;
        }
    }
    return $result;
}