Form Alter in Drupal

Perform alterations before a form is rendered.

One popular use of this hook is to add form elements to the node form.

Note that instead of hook_form_alter(), which is called for all forms, you can also use hook_form_FORM_ID_alter() to alter a specific form.

Syntax: 

Drupal 4.7 – 5	hook_form_alter($form_id, &$form)
Drupal 6 – 8	hook_form_alter(&$form, & $form_state , $form_id)

Example #1: 

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

    if($form_id == 'menu_edit_item') {  
  
    // move submit to the end
    $form['submit']['#weight'] = 5;
    
    $form['additional_title'] = array(
        '#type' => 'textfield',
        '#title' => t('Additional Title'),
        '#maxlength' => 180,
        '#required' => TRUE,
        '#default_value' => $externalLinks->title,
        '#description' => 'Name of the Additional title'
      );
    }
 
}



/* using hook_form_FORM_ID_alter */

Provide a form-specific alteration instead of the global hook_form_alter().
Modules can implement hook_form_FORM_ID_alter() to modify a specific form, rather than implementing hook_form_alter() and checking the form ID, or using long switch statements to alter multiple forms.
Note that this hook fires before hook_form_alter(). Therefore all implementations of hook_form_FORM_ID_alter() will run before all implementations of hook_form_alter(), regardless of the module order.
 
Syntax:

6	hook_form_FORM_ID_alter(&$form, &$form_state)
7 – 8	hook_form_FORM_ID_alter(&$form, &$form_state, $form_id)

Example #2:
function addons_form_menu_edit_item_alter (&$form, &$form_state) {
 
    // move submit to the end
    $form['submit']['#weight'] = 5;
    
    $form['additional_desc'] = array(
        '#type' => 'textfield',
        '#title' => t('Additional Description'),
        '#maxlength' => 180,
        '#required' => TRUE,
        '#default_value' => $externalLinks->title,
        '#description' => 'Name of the Additional Description'
      );
 
}