array problems

I want to build an array dynamically from a database. I have a structure where there is a top level, under which there can be a sub level and a sub-sub level etc... e.g Top Level 1 id =1 parent id =0 Sub Level 1.1 id =2 parent id =1 Sub Level 1.2 id =3 parent id =1 SubSub Level 1.2.1 id =4 parent id = 3 SubSub Level 1.2.2 id =5 parent id = 3 . . SubSub Level 1.2.2 id =n parent id = 3 Sub Level 1.3 id =n parent id =1 Top Level 2 id =n parent id =0 I want to loop through the entire table (for sub levels parent level = id of parent level) and display the details as above. Initally I have a statement selecting the top level (where parent id = 0) $query = "select * from category where parent_id = 0 order by id asc"; $result=$db->query("$query") or die("Select Failed!"); $counter = 1; while($row = mysql_fetch_array($result)){ $main_id[$counter] = $row["id"]; $title[$counter] = $row["title"]; //build array $defined_category[$main_id[$counter]]=$title[$counter]; //search for lower level..... $counter++; } This works fine, my problem is when I try to search for lower levels. I have tried another 'while loop but I get errors! Anyone know a decent solution for this? Andrew