Plaese, help me. How to set attribute for radio button group?

use checkboxes instead of radio buttons or change your application logic (change conditions needed to take an action).

some people might need the functionality radio buttons offer. and just hiding options which are currently not available to the user is not the way to go either. one might want to disable the option and include a title attribute or perhaps even a note on the left explaining why it is disabled and what can be done to solve the issue.
i know there's a 'radio' form element type available with the form api, but there's no real explanation on how to use it in the docs. am i supposed to build an array of these and stick them in the '#options' attribute of the 'radios' element? anyone figure this out?
thanks!

There is a way to set attributes for (or disable) individual radio buttons in Drupal 5, but it is not super straight forward. I've only used this method to disable individual radio buttons, so I'll stick with that as an example, but presumably you could set #attributes for each radio as well.

If you check out the form.inc file, you'll see that the '#options' key is just a shortcut for you. It actually creates individual radio types for each option. So the way you disable individual options is create your own radio types and no #options. Here's an example.

Say you wanted to disable the 'Optional' in this example from the forms api reference (please note that though there is no key given in the #options array, the keys are actually 0 & 1, though I've only done this with associative arrays):

 $form['posting_settings']['comment_preview'] = array(
  '#type' => 'radios',
  '#title' => t('Preview comment'),
  '#default_value' => variable_get('comment_preview', 1),
  '#options' => array(t('Optional'), t('Required')),
);

Instead you would use the following:

 $form['posting_settings']['comment_preview'] = array(
  '#type' => 'radios',
  '#title' => t('Preview comment'),
  0 => array(
    '#type' => 'radio',
    '#title' => t('Optional'),
    '#return_value' => 0,
    '#default_value' => variable_get('comment_preview', 1),
    '#parents' => array('comment_preview'),
    '#spawned' => TRUE,
    '#disabled' => TRUE,
  ),
  1 => array(
    '#type' => 'radio',
    '#title' => t('Required'),
    '#return_value' => 1,
    '#default_value' => variable_get('comment_preview', 1),
    '#parents' => array('comment_preview'),
    '#spawned' => TRUE,
  ),
);

A final couple of notes - If you don't set the '#parents' item correctly, your radios will not act like a radio group. And I don't know what #spawned does, but it is set for each item in the #options list, so I just stuck it in there with a hardcoded true.

For disabling specified checkbox you can use followed function in form_alter hook:

/*
 * Change options for individual checkbox or radio field in the form
 * You can use this function using form_alter hook.
 * i.e. _set_checkbox_option('field_tier_level', 'associate', array('#disabled' => 'disabled'), $form);
 *
 * @param $field_name (string)
 *    Name of the field in the form
 * @param $checkbox_name (string)
 *    Name of checkbox to change options (if it's null, set to all)
 * @param $options (array)
 *    Custom options to set
 * @param $form (array)
 *    Form to change
 *
 * @author kenorb at gmail
 */
function _set_checkbox_option($field_name, $checkbox_name = NULL, $options, &$form) {
    if (isset($form[$field_name]) && is_array($form[$field_name])) {
        foreach ($form[$field_name] as $key => $value) {
            if (isset($form[$field_name][$key]['#type'])) {
                $curr_arr = &$form[$field_name][$key]; // set array as current
                $type = $form[$field_name][$key]['#type'];
                break;
            }
        }
        if (isset($curr_arr) && is_array($curr_arr['#default_value'])) {
            switch ($type) { // changed type from plural to singular
                case 'radios':
                    $type = 'radio';
                    break;
                case 'checkboxes':
                    $type = 'checkbox';
                    break;
            }

            foreach ($curr_arr['#default_value'] as $key => $value) {
                foreach($curr_arr as $old_key => $old_value) { // copy existing options for to current option
                    $new_options[$old_key] = $old_value;
                }
                $new_options['#type'] = $type;  // set type
                $new_options['#title'] = $value;  // set correct title of option
                $curr_arr[$key] = $new_options; // set new options

                if (empty($checkbox_name) || strcasecmp($checkbox_name, $value) == 0) { // check name or set for 
                    foreach($options as $new_key => $new_value) {
                        $curr_arr[$key][$new_key] = $value;
                    }
                }
            }
            unset($curr_arr['#options']); // delete old options settings
        } else {
            return NULL;
        }
    } else {
        return NULL;
    }
}

Or for disabling field this one:

/*
 * Disable selected field in the form(whatever if it's textfield, checkbox or radio)
 * You can use this function using form_alter hook.
 * i.e. _disable_field('title', $form);
 *
 * @param $field_name (string)
 *    Name of the field in the form
 * @param $form (array)
 *    Form to change
 *
 * @author kenorb at gmail
 */
function _disable_field($field_name, &$form) {
    $keyname = '#disabled';

    if (!isset($form[$field_name])) { // case: if field doesn't exists, put keyname in the main array
        $form[$keyname] = TRUE;
    } else if (!isset($form[$field_name]['#type']) && is_array($form[$field_name])) { // case: if type not exist, find type from inside of array
        foreach ($form[$field_name] as $key => $value) {
            if (isset($form[$field_name][$key]['#type'])) {
                $curr_arr = &$form[$field_name][$key]; // set array as current
                break;
            }
        }
    } else {
        $curr_arr = &$form[$field_name]; // set field array as current
    }

    // set the value
    if (isset($curr_arr['#type'])) {
        switch ($curr_arr['#type']) {
            case 'textfield':
            default:
                $curr_arr[$keyname] = TRUE;
        }
    }
}