[Scott MacVicar]

Yes it returns an array, thats how you get data from the database, or you fetch row or fetch object.

use $settinggroupid[LAST_INSERT_ID()] as you said its an array. If your ever curious to what value an array holds you can use print_r($array);
and it will print out all the values of $array.

Anyway you can use $DB_site->insert_id(); to use the abstraction layer if you want.

its part of the abstraction layer which simply uses mysql_insert_id(); which is a nice trait of mysql as it gets the last value of the auto incrememnt column.