Conditional menus based on page title

Question

I'm looking to create and display a separate menu for a specific page. This menu needs to be in the primary location. So for example, on the Home page and the Contact page you would have: Home | About us | Contact. And on the "About us" page, you could show a different menu for example: Menu link 1 | Menu link 2 | Menu link 3. I have been trying for ours to do this with no luck. I literally have been looking at this for so long I don't even know where to start anymore. :( I'm hoping it's something like the code below.. Anyway, any help would be greatly appreciated. Thanks

function my_example_menu() {
  register_nav_menu('example-menu',__( 'Example Menu' ));
}
add_action( 'init', 'my_example_menu' );

wp_nav_menu( array( 
    'theme_location' => 'my-example-menu', 
    'container_class' => 'my-example-menu-class' ) ); 

function my_display_example_menu() {
  if (is_page('about-us')) {
  unregister_nav_menu( 'Primary Navigation' );
  register_nav_menu( 'Example Menu' );
  }
}

//EDIT

Parent theme nav

<?php truethemes_before_primary_navigation_hook();// action hook ?>

<nav role="navigation">
<?php if('true' == $ubermenu):
    wp_nav_menu(array(
        'theme_location' => 'Primary Navigation' ,
        'depth'          => 0 ,
        'container'      => false ,
        'walker'         => new description_walker() ));
else: ?>
<ul id="menu-main-nav" class="sf-menu">
<?php wp_nav_menu(array(
        'theme_location' => 'Primary Navigation' ,
        'depth'          => 0 ,
        'container'      => false ,
        'walker'         => new description_walker() )); ?>
</ul>
<?php endif; //end uberMenu check  ?>
</nav>
<?php truethemes_after_primary_navigation_hook();// action hook ?>

Answer 1

As you can read from the documentation register_nav_menu is used to register a menu in the menu editor. Your code will affect the backend only. Also you should pass a slug like string as the first parameter. If you want to change the display of the menu, you could simple write a condition which will change the parameters of wp_nav_menu. Like the following:

// header.php (or similar)
wp_nav_menu([
  'menu' => (is_page('about-us') ? 'primary-navigation' ? 'example-menu'),
]);

Otherwise you could also register a menu and change the value of the theme_location parameter to make the menu configurable by the user.

Update:

To achieve this via a filter hook

// functions.php
add_filter('wp_nav_menu_args', function ($args) {
  if (is_page('about-us')) {
    $args['menu'] = 'my-custom-menu';
  }
  return $args;
});

Answer 2

1 - Copy the parent theme's header.php into your child theme.

2 - To create custom fields, if you are not too knowledgable of WP, I suggest using the Advanced Custom Fields plugin.

Read the docs on their website. You want to create a select/dropdown or radio type field and have it for all post types and pages.

For each menu that you have, give the custom field a value.

Give the custom field a default value - the one for the menu that you will be using most (so that it'll save you some time)

3 - Give the other pages/posts the other menu.

4 - In your child theme's header.php, change this:

<?php if('true' == $ubermenu):
    wp_nav_menu(array(
    'theme_location' => 'Primary Navigation' ,
    'depth'          => 0 ,
    'container'      => false ,
    'walker'         => new description_walker() ));
else: ?>
 <ul id="menu-main-nav" class="sf-menu">
 <?php wp_nav_menu(array(
    'theme_location' => 'Primary Navigation' ,
    'depth'          => 0 ,
    'container'      => false ,
    'walker'         => new description_walker() )); ?>
</ul>
<?php endif; //end uberMenu check  ?>

To:

<?php 
$menu_to_use = get_field(get_the_ID(), 'meta_key_of_custom_field');
if('true' == $ubermenu):
    wp_nav_menu(array(
    'theme_location' => $menu_to_use, // make sure that the values of the custom field are the names of the menus (e.g. Primary Menu)
    'depth'          => 0 ,
    'container'      => false ,
    'walker'         => new description_walker() ));
else: ?>
 <ul id="menu-main-nav" class="sf-menu">
 <?php wp_nav_menu(array(
    'theme_location' => $menu_to_use, // make sure that the values of the custom field are the names of the menus (e.g. Primary Menu)
    'depth'          => 0 ,
    'container'      => false ,
    'walker'         => new description_walker() )); ?>
</ul>
<?php endif; //end uberMenu check  ?>