WordPress navigation menu

header.php

<div class="main-menu">
    <?php
        wp_nav_menu( array(
            'theme_location' => 'primary',
            'menu_id'     => 'menu',
            'fallback_cb'    => 'ah_fallback_menu'
         ) );
    ?>
</div>

menus.php

<?php 
  function ah_navigation(){
    register_nav_menus( array(
      'primary' => __( 'Primary Menu', 'ah' ),
      'footer_menu'  => __( 'Footer Menu', 'ah' ),
    ) );
  }
  add_action( 'init', 'ah_navigation' );

  //fallback_menu
  function ah_fallback_menu(){
    echo '<ul id="menu">';
    if('page' != get_option('show_on_front')){
      echo '<li><a href="' . home_url() . '/">Home</a></li>';
    }
    wp_list_pages('title_li=');
    echo '</ul>';
  }

?>