php function to show nav menu with example

// 1. registering the menus
    function register_my_menu() {
      register_nav_menu('header-menu',__( 'Header Menu' ));
    }
    add_action( 'init', 'register_my_menu' );

    // adding two menus
    function register_my_menus() {
      register_nav_menus(
        array(
          'header-menu' => __( 'Header Menu' ),
          'extra-menu' => __( 'Extra Menu' )
        )
      );
    }
    add_action( 'init', 'register_my_menus' );

// 2. Displaying the menus in theme
	<?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?>
  		// displaying extra menu in certain page with certain class
  			wp_nav_menu( array( 'theme_location' => 'extra-menu', 'container_class' => 'my_extra_menu_class' ) );