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<?php
    // Break down an instance of wp_nav_menu to its simplest components
    // source from https://codex.wordpress.org/Function_Reference/wp_get_nav_menu_items

    // Get the nav menu based on $menu_name (same as 'theme_location' or 'menu' arg to wp_nav_menu)
    // This code based on wp_nav_menu's code to get Menu ID from menu slug

    $menu_name = 'custom_menu_slug';

    if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $menu_name ] ) ) {
	$menu = wp_get_nav_menu_object( $locations[ $menu_name ] );

	$menu_items = wp_get_nav_menu_items($menu->term_id);

	$menu_list = '<ul id="menu-' . $menu_name . '">';

	foreach ( (array) $menu_items as $key => $menu_item ) {
	    $title = $menu_item->title;
	    $url = $menu_item->url;
	    $menu_list .= '<li><a href="' . $url . '">' . $title . '</a></li>';
	}
	$menu_list .= '</ul>';
    } else {
	$menu_list = '<ul><li>Menu "' . $menu_name . '" not defined.</li></ul>';
    }
    // $menu_list now ready to output
    echo $menu_list;
?>