backdrop_radios.php

<?php

$form['numeric_default_zero'] = array(
  '#type' => 'radios',
  '#title' => "Only numeric keys, starting at 0. #default_value to 0",
  '#default_value' => 0,
  '#options' => array(
    0 => "key: 0",
    1 => "key: 1",
    2 => "key: 2",
  ),
  '#description' => "GOOD: radio 0 is selected",
);

$form['numeric_default_false'] = array(
  '#type' => 'radios',
  '#title' => "Only numeric keys, starting at 0. #default_value to FALSE",
  '#default_value' => FALSE,
  '#options' => array(
    0 => "key: 0",
    1 => "key: 1",
    2 => "key: 2",
  ),
  '#description' => "GOOD: no radio selected",
);

$form['mixed_default_zero'] = array(
  '#type' => 'radios',
  '#title' => "Mixed numeric & alpha keys, starting at 0. #default_value to 0",
  '#default_value' => 0,
  '#options' => array(
    0 => "key: 0",
    1 => "key: 1",
    'a' => "key: a",
    'b' => "key: b",
  ),
  '#description' => "<strong>BAD</strong>: 0 and alpha radios have 'checked' attribute, last one appear as selected",
);

$form['mixed_default_false'] = array(
  '#type' => 'radios',
  '#title' => "Mixed numeric & alpha keys, starting at 0. #default_value to FALSE",
  '#default_value' => FALSE,
  '#options' => array(
    0 => "key: 0",
    1 => "key: 1",
    'a' => "key: a",
    'b' => "key: b",
  ),
  '#description' => "GOOD: no radio selected",
);

$form['alpha_default_zero'] = array(
  '#type' => 'radios',
  '#title' => "Only alpha keys. #default_value to 0",
  '#default_value' => 0,
  '#options' => array(
    'a' => "key: a",
    'b' => "key: b",
  ),
  '#description' => "<strong>BAD</strong>: every radios have 'checked' attribute, last one appear as selected",
);

$form['alpha_default_false'] = array(
  '#type' => 'radios',
  '#title' => "Only alpha keys. #default_value to FALSE",
  '#default_value' => FALSE,
  '#options' => array(
    'a' => "key: a",
    'b' => "key: b",
  ),
  '#description' => "GOOD: no radio selected",
);