How can add a textfield to user/register page form in drupal 7?

Question

I want modify and adding a textfield in my Drupal 7 user/register page. I know that the form is generated by the function user_register_form()

Can i add a textfield in this way?

function bartik_copy_user_login($form, &$form_state) {
  global $user;

  // If we are already logged on, go to the user page instead.
  if ($user->uid) {
    drupal_goto('user/' . $user->uid);
  }

  // Display login form:
  $form['name'] = array('#type' => 'textfield',
    '#title' => t('Username'),
    '#size' => 60,
    '#maxlength' => USERNAME_MAX_LENGTH,
    '#required' => TRUE,
  );

  $form['name']['#description'] = t('Enter your @s username.', array('@s' => variable_get('site_name', 'Drupal')));
  $form['pass'] = array('#type' => 'password',
    '#title' => t('Password'),
    '#description' => t('Enter that accompanies your username.'),
    '#required' => TRUE,
  );
$form['address'] = array('#type' => 'textfield',
    '#title' => t('Your address'),
    '#size' => 60,
    '#maxlength' => 125,
    '#required' => TRUE,
  );

  $form['#validate'] = user_login_default_validators();
  $form['actions'] = array('#type' => 'actions');
  $form['actions']['submit'] = array('#type' => 'submit', '#value' => t('Log in'));

  return $form;
}

Answer 1

I would rather implement hook_form_FORM_ID_alter() to add the form field.

function mymodule_form_user_register_form_alter(&$form, &$form_state, $form_id) {
  $form['address'] = array('#type' => 'textfield',
    '#title' => t('Your address'),
    '#size' => 60,
    '#maxlength' => 125,
    '#required' => TRUE,
  );
}

In this way, the form field will be added to the registration form.

Answer 2

Drupal 7 provides the ability to add custom fields to the User through admin/config/people/accounts/fields. This functionality should already be in core.

Answer 3

Example on how to programatically add fields to the user profile and how to avail them, or not, into the User Registration form.

function MYMODULE_enable() {
  // Check if our field is not already created.
  if (!field_info_field('field_myField')) {
    $field = array(
      'field_name' => 'field_myField', 
      'type' => 'text', 
    );

    field_create_field($field);

    // Create the instance on the bundle.
    $instance = array(
      'field_name' => 'field_myField', 
      'entity_type' => 'user', 
      'label' => 'My Field Name', 
      'bundle' => 'user', 
      // If you don't set the "required" property then the field wont be required by default.
      'required' => TRUE,
      'settings' => array(
        // Here you inform either or not you want this field showing up on the registration form.
        'user_register_form' => 1,
      ),
      'widget' => array(
        'type' => 'textfield',
      ), 
    );

    field_create_instance($instance);
  }
}