d7-form-id-alter-example.php

<?php

// playground.module file.

/**
 * Implements hook_form_FORM_ID_alter().
 */
function playground_form_user_login_block_alter(&$form, &$form_state) {
  $form['preference'] = array(
    '#type' => 'select',
    '#options' => array(
      'test',
      'test1',
      'test2',
      'test3',
    ),
    '#multiple' => TRUE,
  );
  $form['#submit'][] = 'playground_form_user_login_block_sumbit';
}


function playground_form_user_login_block_sumbit($form, &$form_state) {
  print_r($form_state['values']);exit;
}

// After selecting first 3 option and submitting form will give:

Array
(
    [name] => username
    [pass] => password
    [submit] => Log in
    [form_build_id] => form-gwDOjVttTr6yEM1ZmNmUFmAPH3PO2MyFJ2O_c6SqUM0
    [form_id] => user_login_block
    [preference] => Array
        (
            [0] => 0
            [1] => 1
            [2] => 2
        )

    [op] => Log in
)