Preprocess submitted data

Question

I created WordPress module, which adds my custom field in general settings in WordPress Admin. However after submitting whole form in admin settings, I need to modify my field's value before saving to database. How can I do that?

I would prefer to modify my field's data before any default WordPress form validation. Any clues? So far I spent on this hours, and could not google anything.

My code:

Located in my_module's main php file.

function my_module__section(){

    $settings_page_display = 'general';
    $section_id = 'my_module__section';
    add_settings_section(  
        $section_id,
        'Notification information',
        'my_module__section_description',
        $settings_page_display
    );

    $field_id = 'notification_text';
    $field_label = 'Name of customer';

    add_settings_field(
        $field_id,
        $field_label,
        'my_module__settings_fields_render',
        $settings_page_display,
        $section_id,
        [$field_id, 'text']
    );
    register_setting('general', $field_id, 'esc_attr');

}

function my_module__section_description() {
    echo '<p>Some description</p>';  
}

function my_module__settings_fields_render($args) {
    $field_id = $args[0];
    $field_type = $args[1];
    $option = get_option($field_id);
    echo '<input type="' . $field_type . '" id="'. $field_id .'" name="'. $field_id .'" value="' . $option . '" required>';
}

add_action('admin_init', 'my_module__section');

Note:

I am not searching for any frontend / javascript hacks - no exception.

Answer 1

If I got your question right then maybe you could try using the update_option action, which according to the docs "Fires immediately before an option value is updated.". It gets option, old value and new value as parameters. https://developer.wordpress.org/reference/hooks/update_option/

The accepted answer here, Hook if somebody saves plugin options?, might also be relevant for you.

If you feel adventurous, you could add a custom sanitize callback to register_setting and use it to do some funky stuff with the setting value. But this might be a little unorthodox way.

Personally I've found out that digging into the WP source code on trac is also a good way to find useful actions and filters for your custom functions. In this case for example the option.php https://core.trac.wordpress.org/browser/tags/5.1.1/src/wp-includes/option.php