ffdn.org/sites/all/modules/captcha/image_captcha/image_captcha.install

<?php

/**
 * @file
 * Installation/uninstallation related functions for the image_captcha module.
 */

/**
 * Implements hook_requirements().
 */
function image_captcha_requirements($phase) {
  $requirements = array();
  $t = get_t();
  if ($phase == 'install') {
    // _image_captcha_check_setup() is defined in image_captcha.module.
    module_load_include('module', 'image_captcha');
    // Check if the GD library is available and raise an error when not.
    if (_image_captcha_check_setup(FALSE) & IMAGE_CAPTCHA_ERROR_NO_GDLIB) {
      $requirements['image_captcha_requires_gd'] = array(
        'title' => $t('Image CAPTCHA requires GD library'),
        'description' => $t(
          'The Image CAPTCHA module can not be installed because your PHP setup does not provide the <a href="!gddoc">GD library</a>, which is required to generate images.',
          array('!gddoc' => 'http://www.php.net/manual/en/book.image.php')
        ),
        'severity' => REQUIREMENT_ERROR,
      );
    }
  }
  return $requirements;
}

/**
 * On uninstall: remove module variables and clear variable cache.
 */
function image_captcha_uninstall() {
  db_delete('variable')
    ->condition('name', db_like('image_captcha_') . '%', 'LIKE')
    ->execute();
  cache_clear_all('variables', 'cache');
}