PHP: Wordpress navigation menu

gistfile1.php

<?php
// functions.php
add_theme_support('menus'); 
function reg_menu() {
	register_nav_menus(
		array(
			'head-menu' => __('Head Menu')
			)
	);
}
add_action( 'init', 'reg_menu' );
?>
<?php
// inside theme
if (function_exists('wp_nav_menu')){
	wp_nav_menu(array( 
		'container_id'	=> 'menu_box',
		'theme_location' => 'head-menu'
	));
}
?>