how fill select from another select?

[eluser]Asinox[/eluser]
Hi, im new with CI, and i hav some question about it...

i hav a form and this form hav 2 select (categories and subcategories) the subcategories select i need that the values (option) change if i change the categories and then ill select the subcategories from categories...

And the another question is, the select categories i filled with a funcion that make query and bring me all categories, but when i want to insert the data i hav problem because i hav to function calling the same view (i dont know how use 2 function in the same view), i hav 1 function for fill the categories select and another for save the POST data....but the data can't save...

somebody help me please?

Thanks u