Sorting array of arrays problem

hi guys

i need to sort an array of products that has another array of categories inside,
something like

$prod = array();
$data = array();
$cat  = array();
$n = 0;

// $qr is a returned sql query response

 while($ln = mysql_fetch_array($qr))
{
$data['id'] = $ln['product_id'];
$data['name'] = $ln['product_name'];
$data['cat'] = $ln['product_category'];

$prod[$n] = $data;
$n++;

}

return $prod;

now i have to sort this $prod array by categories without making a new sql query, how do i do that?