dropdown with multiple

Hello, I wanted to use dropdown with multiple attribute using helper forms:

PHP Code:

<?= form_dropdown('dpd', array(
        '1' => 'First',
        '2' => 'Second',
        '3' => 'third',
        '4' => 'fourth',
), '2', 'class = "form-control" multiple');?>

PHP Code:

$drpdown = $this->request->getPost('dpd'); 


I choose several values, expect to get something like an array...

but instead I get:

Code:

string '3' (length=1)

Yes, this value also participates in the sample, but it is chosen last. There should be 2 more.

What have I done wrong? Thanks!