How to override form just on one page?

Question

OK so this is my hook form alter function.It is causing all the registration forms on site to be over written which I do not want as I just want it on this page.

 
function special_registration_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'user_register') {
    drupal_set_title(t('Custom registration'));
    $form['firstname'] = array('#type' => 'textfield', 
                               '#title' => t('First Name: *'), 
                               '#required' => TRUE, 
                               '#size' => 45, 
                               '#weight' => - 100,);
    $form['lastname'] = array('#type' => 'textfield', 
                              '#title' => t('Last Name: *'), 
                              '#required' => TRUE, 
                              '#size' => 45, 
                              '#weight' => - 99,);
  }

I only first name and last name to be captured and stored in a different table just on this page.

On other pages I just want the good old fashioned form. Do I still need to change the weight? I know I am missing something elementary.

Answer 1

You just need a check for the current page, using either arg or $_GET['q'].

eg:

function special_registration_form_alter(&$form, $form_state, $form_id) {
if ($_GET['q'] !== 'whatever/path' ) { return false; }
..rest of code..
}

Answer 2

If you want to restrict your form alterations to a specific page, you can simply add a check for the page to your form id check, e.g.:

function special_registration_form_alter(&$form, $form_state, $form_id) {
  // Alter the registration form, but only on 'user/register' pages
  if ($form_id == 'user_register' && 'user' == arg(0) && 'register' == arg(1)) {
    // snipped alteration code
  }
}