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

Adding a node:

function add_node( parent, name )
{
    $parent_info =
        SELECT *
        FROM tree
        WHERE Id = $parent;
    
    INSERT INTO tree(Id, Name) VALUES( NULL, $name );
    
    $last_id = LAST INSERT ID;
    UPDATE tree
        SET
            ParentId = $parent,
            PathString = $parent_info['PathString'] . $last_id . "/",
            Depth = $parent['Depth'] + 1
        WHERE Id = $last_id
}