Customize existing menu item

Question

I am trying to manipulate the existing primary menu with a plugin.

The result should be: if the plugin is activated, it edits the 'Login' button to be conditional:

if something is true
     show profile picture and name
else
     show login button

There is still a long way to that result, right now I'm trying to generally manipulate the main menu with a custom Walker class

Here is my code:

add_action( 'wp_loaded', 'edit_that_shit');

function edit_that_shit(){
    wp_nav_menu(
        array(
            'theme_location' => 'primary',
            'menu' => 'Primary Navigation',
            'walker' => new Custom_Walker()
        )
    );
}


class Custom_Walker extends Walker_Nav_Menu
{
     public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 )
     { ... }
}

But the only thing that happens is, that WordPress creates a new menu beside the main menu, with the same items on top of the page.

Answer 1

Given that you want to include a picture in the wp_nav_menu output, your best bet is to use the wp_nav_menu_items filter, as suggested in the comments. This will allow you to do a search and replace on the html.

add_filter ('wp_nav_menu_items','wpse229358_filter_menu');
function wpse229358_filter_menu ($items,$args) {
  $search  = ''; // html string of button
  $replace = ''; // html string of picture+name
  if (yourcondition) {
    str_replace ($search,$replace,$items);
    }
  }