form_alter in drupal module

form_alter is basically the way we can change the defaults or behaviour of any form.

hook_form_alter(&$form, &$form_state, $form_id)

It performs alteration before rendering. It is also used to add form elements to the node form. The node object can be accessed at $form['#node'], when altering the node form.

$form: It is the nested array of form elements.

$form_state: A keyed array containing the current state of the form.

$form_id: String representing the name of the form itself.

Lets continue with an example of adding a content type. When you navigate to Structure >> Content type >> Add content type and scroll down you can find Display settings

By default, the value has been checked. Lets uncheck it by using form_alter in a module.
Create a folder in sites >> all >> module named as mymod. Inside that folder create two files mymod.info and mymod.module.
Include following lines of code in mymod.info

name = mymod
description = This description for mymod
core = 7.x

name : it is the name of the module.
description : it is the description for the module

core : It is the version of drupal

Include following lines of code in mymod.module

<?php
function mymod_form_alter(&$form, &$form_state, $form_id){
if($form_id == 'node_form'){
$form['display']['node_submitted']['#default_value'] = 0;
}
}

In the above code, default value will be set to 0 (i,e. uncheck) for the node id node_form

We can also change the description which is appearing below the check box by including the following lines

$form['display']['node_submitted']['#description'] = t("This is the description");

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