Add option in select and set_select than

Hello, everyone. I`m new to Codeigniter and PHP too. So here is my problem. I have two dropdowns each with values - '1', '2' and '3'(set_selected).  By default is '3' from the first and disables the second dropdown, also prepending to 2nd a new disabled empty option and remove it if '1' or '2' is selected from the 1st than(using JQuery) Any suggestions how to set_select the new option in 2nd dropdown or maybe I`ll need to change my concept? Thanks