Using form_alter to disable ONE option of a select field

Looks like you're not going to be able to do it in 6.x but there's a 7.x patch to do it (http://drupal.org/node/284917).

I'm surprised that didn't get implemented in the 6.x FAPI. The only options I can see are to try a jquery solution or to remove the option entirely.

You want to add an attribute of #pre_render to your API call that builds the #options. #pre_render takes an array of functions that should be called before the form is output. You'd then declare a function to manipulate the particular options that need it. Example:

function make_option21_disabled( $form_select ) {
  $form_select[21]['#attributes']['disabled']='disabled';
  return $form_select;
}

See #pre_render in the forms API documentation at http://api.drupal.org/api/drupal/developer--topics--forms_api_reference....

You can use next code as example:
$form['some_value'] = array(
'#type' => 'select',
'#title' => 'Select with disabled option at the top',
'#options' => array(
t('- Please select -') => array(
'one' => t('First option'),
'two' => t('Second option'),
)
)
);

operating on the html just prior to display, e.g.:

...
  	$form['f2']['consumptionProfile'] =
      array(
        '#title' => t('Type of Spending'),
        '#type' => 'select',
        '#options' => $options,
        '#default_value' => $row->consumptionProfile,
  			'#attributes' =>
  				array(
  					'class' => 'monteCarlo',
  				),
  			'#post_render' => array('_ESPlannerBasicPlanningMethod_callback'),
  		) ;

...

function _ESPlannerBasicPlanningMethod_callback(
	$theContent,
	$theElement)
{
	$map =
		array(
			1 => ESPlannerBasic_permission_monteCarlo_aggressive_paid,
			2 => ESPlannerBasic_permission_monteCarlo_cautious_paid,
			3 => ESPlannerBasic_permission_monteCarlo_conservative_paid,
		) ;
	foreach ($map as $k => $p)
	{
		if (!user_access($p))
		{
			$xxx = sprintf('/(value="%d")/', $k) ;
			
			$theContent = preg_replace($xxx, '$1 disabled="disabled"', $theContent) ;
		}
	}
	
	return $theContent ;
}

The code snipped above, checks for user access to some of the features of our product and disables them in the menu if the user doesn't have access.

Best,

Dick Munroe

$form['bind_slider_to'] = array(
          '#type' => 'select',
          '#title' => t('Field to have a Slider'),
          '#options' => array(
             'option1' => t('Option 1'),
             'option2' => t('Option 2'),
             t('Disabled option') => array(),
             'option3' => t('Option 3'),
             t('Disabled option 2') => array(),
          ),
);

20131102 - removed by author

In case anyone runs up against this, I came up with a solution of using a #post_render, string replacement and a non-standard element in the select array to disable specific options. Here is an example:

$options = array('Zero', 'One', 'Two', 'Three', 'Four', 'Five', 'Six');
$ids_to_disable = array(2,5,6);

$form['select'] = array(
  '#type' => 'select',
  '#options' => '#options',
  '#post_render' => array('disable_ids'),
  '#ids_to_disable' => $ids_to_disable
);

function disable_ids($rendered, $element) {
  if (empty($element['#ids_to_disable']) === true) {
    return $rendered;
  }
  
  $search = array();
  $replace = array();
  foreach ($element['#ids_to_disable'] as $id) {
    $search[] = '<option value="' . $id. '">';
    $replace[] = '<option value="' . $id. '" disabled="disabled">';
  }
  $rendered = str_replace($search, $replace, $rendered);
  
  return $rendered;
}

This is how I do it:

$form['happy'] = array(
    '#title' => t('Are you happy?'),
    '#type' => 'checkboxes',
    '#options' => array(
      'yes' => t('Yes'),
      'no' => t('No'),
    ),
);
$form['happy']['yes'] = array(
    '#attributes' => array(
      'class' => array('always-happy'),
    ),
);
$form['happy']['no'] = array(
    '#disabled' => TRUE,
);

I hope this is helpful...

This contrib module can help

https://www.drupal.org/project/form_options_attributes