Drupal: Creating a simple admin configuration page

It's very easy to create an admin configuration page in Drupal with the help of system_settings_form(), this function adds a Save Configuration button and handle the saving of the variables, you don't have to create a submit function  and do a variable_set to save your settings.

This are the functions that we need:

1. hook_permission(); // Create your own permission using this function.
2. hook_menu(); // Create your own menu using this function.
3. drupal_get_form(); // Create your own form.
4. your_own_form_function(); // This will be your form name that we will pass on drupal_get_form().
5. variable_get(); // This function will get the saved configuration on the database.
6. system_settings_form();
7. your_own_form_function_validate(); // You can add this if you want to do custom validation for your forms.

(I expect that you already know how to create a .info and .module file, if not then I suggest that you learn that first before proceeding...)

First we need to create a permission so no one can just edit your configuration.

 /**
 * Implements hook_permission().
 */
function MODULENAME_permission() {
  return array(
      // You can check if a user has an access permission by using user_access('administer your module'), 
      // this will return a boolean value.
      'administer your module' => array(
        'title' => 'Administer your module',
        'description' => 'This is your module permission',
      ),
    )
  ;
}

After we create a permission, we will now create a page. Most of the config path is located at admin/config so it is best to do the same.

 /**
 * Implements hook_menu().
 */
function MODULENAME_menu() {
  $items['admin/config/YourPathHere'] = array(
    'title' => t('My Module Config'),
    'page callback' => 'drupal_get_form', // This will call drupal_get_form() function and pass in the page arguments.
    'page arguments' => array('any_function_name_admin_config'), // This will be the name of your function form.
    'access arguments' => array('administer your module'), // This will check if the user have the correct permission.
    'file' => 'MODULENAME.admin.inc', // We can put our code in different file to make our code organized.
  );

  return $items;
} 

If you specify a file path on your hook_menu then this function should be added to that file. Don't forget to add <?php on the first line of your config file.

Visit Form API Reference for the list of properties that you can do with the form.

 /**
 * My Module configuration form.
 */
function any_function_name_admin_config($form, &$form_state) {
 $form['mymodule_simple_text'] = array(
  '#type' => 'textfield',
  '#title' => t('Text field'),
  '#description' => t('This is the description of the text field.'),
  '#required' => TRUE,
  '#default_value' => variable_get('mymodule_simple_text', ''),
 );

 return system_settings_form($form);
} 

You can add your own validation by adding _validate to your admin config function name. Use form_set_error to return an error message and stop submitting the form.

 /**
 * Implements hook_form().
 */
function any_function_name_admin_config_validate($form, &$form_state) {
  if (empty($form_state['values']['mymodule_simple_text'])) {
    form_set_error('mymodule_simple_text', t('Text field cannot be empty.'));
  }
}