Id | ParentId | PathString  | Name         | Depth
---+----------+-------------+--------------+------
87 |        2 | /87/        | Countries    |  2
88 |       87 | /87/88/     | Belgium      |  3
89 |       87 | /87/89/     | Netherlands  |  3
91 |       89 | /87/89/91/  | Business     |  4
92 |       89 | /87/89/92/  | Economy      |  4 
90 |       87 | /87/90/     | Germany      |  3

Selecting a subtree:

function select_subtree( id )
{
    $info =
        SELECT PathString
        FROM tree
        WHERE Id = $id;

    $tree_elements =
        SELECT *
        FROM tree
        WHERE PathString LIKE "$info%";

    return $tree_elements;
}