How to extend a form in Drupal

It's so easy to extend a exists form in drupal,

we just use hook_form_alter to do that

function example_form_alter(&$form, &$form_state, $form_id) {

if ( $form_id == 'user_register_form') {

$form['js_example_form']['fullname'] = array(

'#type' => 'textfield',

'#title' => t('Full Name'),

'#element_validate' => array('example_fullname_validate'),

);

$form['#submit'][] = 'example_fullname_submit';
//$form['#validate'][] = 'example_fullname_validate';

}

}

Create a method for validate new form element value

function example_fullname_validate($form, &$form_state) {
$fullname = $form_state['values']['fullname'];
if (empty($fullname)) {
form_set_error('fullname', 'Fullname can not be empty.');
}
}

Save value of fullname

function example_fullname_submit($form, &$form_state) {
$fullname = $form_state['values']['fullname'];
//save fullname here
}

http://www.chromaticsites.com/blog/drupal-tutorial-form-overrides-and-element-specific-validations/