Altering other forms

Drupal's Form API does not just provide a way to create forms. There are ways to alter forms through a custom module that allows you to manipulate the core and contributed forms. Using this technique, new elements can be added, default values can be changed, or elements can even be hidden from view to simplify the user experience.

The altering of a form does not happen in a custom class; this is a hook defined in the module file. In this recipe, we will use the hook_form_FORM_ID_alter() hook to add a telephone field to the site's configuration form.

Getting ready

This recipe assumes that you have a custom module to add the code to.

How to do it...

1. In the modules folder of your Drupal site, create a folder named mymodule.

2. In the mymodule folder, create a mymodule.info.yml, containing the following code:

name: My module 
description: Custom module that uses a form alter 
type: module 
core: 8.x 
 

3. Next, create a mymodule.module file in your module's directory:

<?php 

/** 
...