Salvare variabile da un dropdown in un form

$form['protocol_value'] = array(
'#title' => t('Protocol'),
'#type' => 'select',
'#description' => 'Select the protocol to be used (HTTP or HTTPS)',
    '#default_value' => variable_get('protocol_value', 'http://'),
'#options' => array(t('http://'), t('https://')),
);