Drupal 7 Module Development

Chapter 5
Drupal also offers a Form API element of type 'hidden', should
developers prefer to use it.
Form API makes form building incredibly simple, but right now this form has two
problems. First, the module should provide some reasonable default settings for
system administrators. Second, when you submit the form, none of the submitted
data is actually handled in any way. Let's take a brief detour from form handling
and look at how Drupal manages persistent system data.
Managing persistent data
Drupal provides a mechanism by which data, which needs to persist semipermanently
(like system settings), can be saved and retrieved. These items are
somewhat confusingly referred to as 'variables' (we will refer to them specifically
as persistent variables from here on to avoid confusion). Persistent variables are
stored in a database table, keyed by a unique name provided by the module that
implements them.
Persistent variables are saved using variable_set(), and retrieved using
variable_get(). These variables can be any type of data that a developer needs, be
it a simple string, an associative array, or a PHP object. The Drupal API for setting/
getting them takes care of all the serialization/unserialization that is necessary
behind the scenes.
variable_get() can also provide a default value, which is useful for situations
where you need a variable which has not already been set, for instance, after a
module is installed for the first time. We can use this to our advantage in our
configuration form as shown in the following snippet:
$form['user_warn_e-mail_subject'] = array(
'#type' => 'textfield',
'#default_value' => variable_get('user_warn_e-mail_subject',
'Administrative Warning'),
'#title' => t('Warning e-mail subject'),
'#size' => 40,
'#maxlength' => 120,
'#required' => TRUE,
'#description' => t(
'The subject of the e-mail which will be sent to users.'),
);
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