How to add checkbox to existing administration page

Question

For a custom module i would love to add a checkbox to "BANDWIDTH OPTIMIZATION" under "admin/config/development/performance". I know how to add a new admin page but this is just one checkbox and i think it would fit more there.

Can anyone tell me how to do that?

Answer 1

To alter a form implemented by another module, you just implement hook_form_alter(). You probably need to change the #weight property of the form element you are adding, to avoid it is rendered in the wrong place.

function mymodule_form_system_performance_settings_alter(&$form, &$form_state) {
 // Add the checkbox to $form.
}

Answer 2

To add another item on the performance settings form you must implement the hook_for_alter or the hook_form_FORM_ID_alter.

To get the form id you just need to inspect the form an get the id attribute of the form tag:

In your custom module you will have the form alter function like this:

function mymodule_form_system_performance_settings_alter(&$form, &$form_state) {
   $form['bandwidth_opt'] = array(
      '#type' => 'checkbox',
      '#title' => t('BANDWIDTH OPTIMIZATION'),
    );

   return $form;
}

Notice that the hyphens of the element id in the HTML are replaced by underscores in the function declaration.

Another good tip is the Form API reference page.