I want to connect my Form table with checkboxes

This is my code for the Severities Checkbox. It shows me a checkbox with the values of Emergency, Alert, Warning e.t.c and i want when someone check one of the values and press the submit button i want: e.g if i check the warning checkbox i want to see only the warning messages: Here is the code of the checkboxes:

$form['blablabla'][$key] = array(
    '#type' => 'select',
    '#options' => drupal_map_assoc(array('', t('Emergency'), t('Alert'), t('Critical'), t('Error'), t('Warning'), t('Notice'), t('Info'), t('Debug'))),
    '#default_value' => variable_get($key),
  );

And here follows the code with my table :

/**
   * Table with Errors
   */
  // Begin building the query.
  $query = db_select('watchdog', 'th')
    ->extend('PagerDefault')
    ->orderBy('wid')
    ->fields('th')
    ->limit(80);

  // Fetch the result set.
  $result = $query->execute();

  // Loop through each item and add to the $rows array.
  foreach ($result as $row) {
    $Severities = unserialize($row -> variables);
    //if ($key == 'warning') {
      if($Severities['%type'] == 'Warning')  {
        $rows[] = array(
          $row -> wid,
          $Severities['%type'],
          $Severities['!message'] . ' in '  . $Severities['%function'] . ' (line ' . $Severities['%line'] . ' of ' . $Severities['%file'] .  ' ).',
        );
      }
    }
 // }
    $header = array('ID', 'Severity', 'Message');
    $form['table'] = array(
      '#theme' => 'table',
      '#header' => $header,
      '#rows' => $rows,
    );