check "checkbox is checked" before submit
All you need are Form API's #states.

function MYMODULE_MYFORM($form, &amp;$form_state) {

  $form['radio_buttons'] = array(
    '#type'   =&gt; 'radios',
    '#options' =&gt; array(0 =&gt; t('Hidden'), 1 =&gt; t('Visible')),
  );
  $form['submit_button'] = array(
    '#type'   =&gt; 'submit',
    '#value'  =&gt; t('Click Here!'),
    '#states' =&gt; array(
      'visible' =&gt; array(
        ':input[name="radio_buttons"]' =&gt; array('value' =&gt; 1),
      ),
    ),
  );

  return $form;
}