Question about form_dropdown() function in the form helper

[eluser]Unknown[/eluser]
I want to know if i can do something like this:

Code:

$options = array(
     '1' => 'test',
     '2' => 'test',
     '3' => 'test',
     '4' => array(
          '5' => 'test',
          '6' => 'test',
          '7' => array(
               '8' => 'test'
          )
     ),
);

echo form_dropdown('categories', $options);

and the output will be:
test
test
test
-test
-test
--test
Thank you!!!