let me guess what you mean ...
if you want to pass a value through a form submit (this means, the value has been selected in a form previous to the actual one), you can use a hidden input:
<form ...>
... new form ...
(here comes the value that you want to conserve🙂
<input type=hidden name=my_dropdown_value value="<?=$my_dropdown_value ?>">
<input type=submit ... of new form ... >
</form>
