Change form data after submit in drupal

in some content type form I added a checkbox. When this checkbox is checked I want to remove some of the submitted data.

To do so I created a custom module (my_module.module):

function my_module_form_alter(&$form, &$form_state) {
    // ...
    $form['#submit'][] = 'my_module_form_alter_submit';
}

function my_module_form_alter_submit($form_id, $form_values) {
    drupal_set_message(t('Submit Function Executed!'));
}

How can I tell this module to refer only to the form of a certain containt type? And how can I remove data when it is submitted?

Solution

Assuming you are altering a node edit form, you can either conditionally add the submit callback such as (in your hook_form_alter):

if(isset($form['#node']) && $form['type']['#value'] == 'page') {
    $form['#submit'][] = 'my_module_form_alter_submit';
}

or, you could check the $form argument in the submit callback in a similar fashion.

You are missing the third argument to the hook_form_alter which should be $form_id, and your submit callback should take the arguments such as:

function my_module_form_alter_submit($form, &$form_state) { ... }