If all you want is to redirect the users to the same site, when they follow a link that takes them to http://www.example.com/external, then you can implement hook_menu() using code similar to the following one:
function mymodule_menu() {
$items = array();
$items['external'] = array(
'title' => 'Redirect',
'page callback' => 'mymodule_redirect',
'access callback' => TRUE,
'type' => MENU_CALLBACK,
);
return $items;
}
function mymodule_redirect() {
drupal_goto($url, array('external' => TRUE));
}
If the URL to which the users are redirected depends from a value passed in the URL, then you can use code similar to the following one:
function mymodule_menu() {
$items = array();
$items['external/%'] = array(
'title' => 'Redirect',
'page callback' => 'mymodule_redirect',
'page arguments' => array(1),
'access callback' => TRUE,
'type' => MENU_CALLBACK,
);
return $items;
}
function mymodule_redirect($id) {
// Calculate $url basing on the value of $id.
drupal_goto($url, array('external' => TRUE));
}