在代码点火器中使用form_multiselect选择的选项

I am using code igniter's form_multiselect for my profile form. I want to show the selected value in select box after submit. Used code is like this:

form_multiselect('brand[]', $brand_list,'id="brand"'); 

I have tried set_multiselect but doesn't work. can any one help!

Pass the third parameter an array of selected values like:

form_multiselect('brand[]', $brand_list, array('small', 'medium'), 'id="brand"');