The values that you posted should still be in the POST array do a print_r($POST); and you will obtain all the values contained within in this array.
If it is there just call $POST['countries'] - assuming that you posted the value with the name countries.
Although to get it selected again id probably use javascript, this is where i cant help ya im afraid, as i think if u include the word selected within the option tag, thats the value the select will show.
hope this helps