$categories = array();
$mainCategories = mysql_query("SELECT id, name FROM categories WHERE parent=0", $conn);
while($ds = mysql_fetch_assoc($mainCategories))
{
    $categories[] = array(
       "Name" => $ds["name"],
       "Children" => GetChildren($ds["id"])
     );
}

function GetChildren($parent)
{
   $children = array();
   $childCats = mysql_query("SELECT id, name FROM categories WHERE parent='".$parent."'");
   while ($cat = mysql_fetch_array($childCats)) {
     $children[] = array(
       "Name" => $cat["name"],
       "Children" => GetChildren($cat["id"])
     );
   }
   return $children;
}