How To set 'selected' tag in a Form Select Options Array

Hey @themetman,

If you are actually looking for a way to set the default value, then simply do this:

$form['month'] = array(
  '#type' => 'select',
  '#title' => ('Select Month or The Full Year'),
  '#options' => array(
    '01' => t('January'),
    '02' => t('February'),
    '03' => t('March'),
    '04' => t('April'),
    '05' => t('May'),
    '06' => t('June'),
    '07' => t('July'),
    '08' => t('August'),
    '09' => t('September'),
    '10' => t('October'),
    '11' => t('November'),
    '12' => t('December'),
    '13' => t('The Full Year'),
  ),
  '#default_value' => '10',
);

If you are looking to add other attributes (such as classes/ID etc.) to '#type' => 'select' form elements, then read on...

@BWPanda's answer above adds the selected="selected" attribute in the <select> tag instead of an individual <option> tag. What you'd want is something like this instead:

$form['month'] = array(
  '#type' => 'select',
  '#title' => ('Select Month or The Full Year'),
  '#options' => array(
    '01' => t('January'),
    '02' => t('February'),
    '03' => t('March'),
    '04' => t('April'),
    '05' => t('May'),
    '06' => t('June'),
    '07' => t('July'),
    '08' => t('August'),
    '09' => t('September'),
    '10' => t('October'),
    '11' => t('November'),
    '12' => t('December'),
    '13' => t('The Full Year'),
  ),
);
$form['month']['10']['#attributes']['selected'] = 'selected';

...or if you used actual numbers instead of strings for the options keys, then:

$form['month'][10]['#attributes']['selected'] = 'selected';

Unfortunately, the above only works for '#type' => 'radios' or '#type' => 'checkboxes' - it does not work for '#type' => 'select'. It is simply not supported by the Form API in Drupal/Backdrop. See this thread in stuckoverflow.com or this article for a way that involves some theming work.

PS: something that does work that way in the Form API, but is not (well-)documented, is adding #description to each individual option (again, for radios/checkboxes only). We have recently done that here in order to add better help text for the "Row width behavior" option in the settings of Flexible Layout Templates.

...alternatively, there is the Form Options Attributes contrib module that allowed this in Drupal, but it has not been ported to Backdrop yet. Lucky that most D7 modules work almost as-is with Backdrop :) ...so until it is properly ported, here's what's worked for me:

1. Download the latest 7.x version of the module, and extract it into your modules directory.

2. Edit the form_options_attributes.info file, so that it looks like this:

name = Form Options Attributes
description = Allows form options for select elements to have individual attributes.

backdrop = 1.x
type = module

3. Enable the module.

4. In your form, do something like this:

$form['month'] = array(
  '#type' => 'select',
  '#title' => ('Select Month or The Full Year'),
  '#options' => array(
    '01' => t('January'),
    '02' => t('February'),
    '03' => t('March'),
    '04' => t('April'),
    '05' => t('May'),
    '06' => t('June'),
    '07' => t('July'),
    '08' => t('August'),
    '09' => t('September'),
    '10' => t('October'),
    '11' => t('November'),
    '12' => t('December'),
    '13' => t('The Full Year'),
  ),
  '#options_attributes' => array(
    '10' => array(
      'selected' => 'selected',
    ),
  ),
);

...or for simplicity (if not adding multiple other attributes to the options):

$form['month'] = array(
  '#type' => 'select',
  '#title' => ('Select Month or The Full Year'),
  '#options' => array(
    '01' => t('January'),
    '02' => t('February'),
    '03' => t('March'),
    '04' => t('April'),
    '05' => t('May'),
    '06' => t('June'),
    '07' => t('July'),
    '08' => t('August'),
    '09' => t('September'),
    '10' => t('October'),
    '11' => t('November'),
    '12' => t('December'),
    '13' => t('The Full Year'),
  ),
);
$form['month']['#options_attributes']['10']['selected'] = 'selected';