Using a PHP class defined in the module file inside the implementation of hook_install()
No need, it's already loaded. Check http://api.drupal.org/api/drupal/includes--module.inc/function/module_enable/7

if ($existing-&gt;status == 0) {
  // Load the module's code.
  drupal_load('module', $module);
  module_load_install($module);
  // …
}