Problem with custom dropdown default value

Question

I've written a module that allows users to alter the display of a Date/Time object by allowing them to change their timezone via a front-end block.

It does it's primary function properly, changes the time display as expected, however, what is supposed to happen is that once you pick a timezone, the page reloads, and the dropdown form will default to the timezone you selected. Except it doesn't, it stays on a single default value

The array is the #options list for the dropdown item, and the single item underneath is the #default_value key for that dropdown.

As you can see, the default value should be set to Australia/Melbourne, but in the dropdown it's not, it's set to America/Creston Mountain time.

I cannot figure out what's going wrong, have I done the default value code incorrectly or something?

EDIT

This is the code for the select element

$timezone = mymodule_timezone_get_user_timezone();
$options = variable_get('mymodule_timezone_select', system_time_zones());
$options = array_filter($options);

// doing a dpm($timezone) comes back with Australia/Melbourne
// doing a dpm($options) comes back with the arrow shown above

$form['timezone_select'] = array(
  '#type' => 'select',
  '#title' => '',
  '#options' =>  $options,
  '#default_value' => $timezone,
  '#prefix' => t('<div class="description">Please select your timezone from the drop down box below and click update to convert your viewing times accordingly.</div>'),
);
$form['submit'] = array(
  '#type' => 'submit',
  '#value' => t('Update') ,
);

And the code referenced in this snippet

function mymodule_timezone_get_user_timezone() {
  $retrieved_timezone = session_cache_get('mymodule_timezone_user_timezone');
  if (empty($retrieved_timezone)) {
    $retrieved_timezone = drupal_get_user_timezone();
  }
  return $retrieved_timezone;
}

The variable for 'mymodule_timezone_select' is just from the default system_time_zones() but the admin has been given the option to delete ones they don't want the user to see (since they are several hundred options by default)

Answer 1

Are you providing #default_value when loading form? If yes, you need to make sure that you're providing what is selected by user first. Check example below:

global $user;
// Check if user has set the timezone,
// if yes use that else set to default 'America/Creston'

$timezone = isset($user->timezone) ? $user->timezone : 'America/Creston';
$form['timezone'] = array(
  '#type' => 'select',
  '#title' => 'Select Timezone',
  '#options' => $options,
  '#default_value' => $timezone,
);

As per Form API Reference Documentation, it says #default value is:

The value of the form element that will be displayed or selected initially if the form has not been submitted yet.

Answer 2

Managed to figure out what was wrong, and my own apologies to those who tried to help me, it turns out that the culprit was with the caching settings for my custom block, which wasn't something I'd included in the description of the question.

Originally my block_info code looked like this

function mymodule_timezone_block_info() {
    $blocks['mymodule_timezone'] = array(
        'info' => t('Timezone'),
        'cache' => DRUPAL_CACHE_PER_USER,
    );
    return $blocks;
}

Which should in my mind have been the correct setting, since I wanted it to cache the users response. The weird part is that it did work locally like this, and the same code even worked on another site, on the same server, with the same drupal caching/session caching settings etc.

The correct solution was to do this instead

function mymodule_timezone_block_info() {
    $blocks['mymodule_timezone'] = array(
        'info' => t('Timezone'),
        'cache' => DRUPAL_NO_CACHE,
    );
    return $blocks;
}

Again, apologies to those who tried to help and couldn't, it's my fault for not including all parts of the block setup in the original request.