flush_rewrite_rules() not working with update_option_{$option}

Question

Will someone please explain to me why flushing doesn't work in my case. I added a custom settings page with a default value, but it doesn't work when I hit save changes. I use the admin_init hook to register settings like this.

register_setting(
                'my_general', 
                'my_general_settings', 
                array(
                    'default'   => 'hello',
                ) 
            );

I added sections and fields, and those work. Now I am having a problem with this. If I update or save the settings, the update_option_my_general_settings doesn't work. It isn't flushing the rules. Here is the function code I used inside this hook.

if ( $old_value === 'hello' || $value === 'hello' ) {
                // Flush rules
                flush_rewrite_rules();
            }

Remember I am supplying these arguments to the function $old_value, $value, $option

The filter mod_rewrite_rules adds the data to .htaccess after I manually refresh the permalinks inside settings. But the flush_rewrite_rules() doesn't seem to be refreshing the permalinks automatically.

What I want is that when the user updates the settings, the flush_rewrite_rules should be populated.

More Info

I basically want to add headers like HSTS, XSS through the WordPress filter mod_rewrite_rules. I know I can add these headers by using the WordPress filter 'wp_headers, but the filter doesn't work in some cases. So I created a settings page. Inside which I created sections and fields. Whenever the user saves the settings I registered with register_setting(), the mod_rewrite_rules filter is called to add necessary data to the .htaccess file. As we will add these headers programmatically, we need to flush the rewrite rules. So, what I did, I found this hook 'update_option{$option}'. The callback function of this hook stores the flush_rewrite_rules() function. Whenever the user sets the value hello, the flushing should occur so the data inside the filter will be added to the .htaccess file. I can't post the code because if I do so, the question will be more code than a question. So I tried to keep the question short and to the point.

Here is the code I think would matter. I skipped the settings fields and sections HTML.

<?php 
add_action('admin_menu',  'add_settings_page' );
add_action('admin_init', 'register_settings_sections_and_fields' );
add_action( 'update_option_hello_general_settings',  'hello_flush_rewrite_rules' , 10, 3 );

function add_settings_page()
{
    add_options_page('Hello Settings', 'Hello', 'manage_options', 'hello',  'hello_settings_markup'  );
}

function register_settings_sections_and_fields()
{
    register_setting(
        'hello_general', 
        'hello_general_settings', 
        array(
            'default'   => 'filter',
        ) 
    );

    add_settings_section( 'hello_general_section', '', null, 'hello_general' );
    add_settings_field( 'hello_general_settings_method', 'Select Method', 'hello_general_settings_method_callback', 'hello_general', 'hello_general_section' );
}

function hello_settings_markup()
{
?>
    <form method="post" action="options.php">
        <?php
            settings_fields( 'hello_general' );
            do_settings_sections( 'hello_general' );
            submit_button();
        ?>
    </form>
<?php
}

function hello_flush_rewrite_rules( $old_value, $value, $option )
{
    if ( $old_value === 'hello' || $value === 'hello' ) {
        // Flush rules
        flush_rewrite_rules();
    }
}

Note: The data gets added to .htaccess after I manually update the permalinks through the dashboard.

I really appreciate any help you can provide.

Answer 1

flush_rewrite_rules() should be called very early within the Wordpress initialization. The reason is that the entpoints cannot be changed afterwards.

A solution could look like this:

function updateMySlugOption($old_value, $value, $option) {
    if( $old_value != $value ) {
        set_transient('update_slugs', 1);
    }
}
add_action( 'update_option_mySlug', 'updateMySlugOption', 10, 3);

function update_slugs() {
    if( get_transient('update_slugs') ) {
        flush_rewrite_rules();
        delete_transient('update_slugs');
    }
}
add_action('init', 'update_slugs');