load a select tag with values ​​of DB action is done if the user has changed the value of another select tag

load a select tag with values ​​of DB action is done if the
user has changed the value of another select tag

My problem is to fill the opions of the first select tag values
​​of the DB, action is done if the user has changed the value
of another tag select. i use php5 with CodeIgniter framework. Thank you in
advance.