How to register menu?

Hello,

long time ago I made this PHP code to show menu items with their thumnails.

<?php
    function get_the_post_thumbnail_src($img)
{
  return (preg_match('~\bsrc="([^"]++)"~', $img, $matches)) ? $matches[1] : '';
}
// 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 = 'menu-homepage'; // change this if necessary
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;
        $mid = $menu_item->object_id;
        if(has_post_thumbnail($mid)) {
            $thumbnail = get_the_post_thumbnail( $mid, 'thumb' );
            $image_url = get_the_post_thumbnail_src(get_the_post_thumbnail( $mid, 'thumb' ));
        }
        $image_url2=substr($image_url, -17);
        if(!$i)
        $i=1;
        else
        $i++;
        if($i==1 or $i==4 or $i==7)
        $styl="style='clear:both'";
        else
        $style="";
        if($image_url2=="pozadiPrazdne.png")
        $menu_list .= '<div class="liWrapperMaly".'.$styl.'"><a href="' . $url . '"><li style="background-image:url(\''.$image_url.'\');"><a href="' . $url . '"><h2>' . $title . '</h2></li></a></div>';
        else
        $menu_list .= '<div class="liWrapper".'.$styl.'"><a href="' . $url . '"><li style="background-image:url(\''.$image_url.'\');"><h2>' . $title . '</h2></li></a></div>';
    }
    $menu_list .= '</ul>';
 } else {
    $menu_list = '<ul><li>Menu "' . $menu_name . '" not defined.</li></ul>';
 }
 // $menu_list now ready to output
 echo $menu_list;
 ?>

Now I want to display another menu by same method, so I simply change this line

$menu_name = 'menu-homepage';

to this line

$menu_name = 'new-menu';

In functions.php I registered new-menu

register_nav_menu('new-menu', 'New Menu');

New Menu is created in WordPress administration, but if I execute this PHP code it writes me this message “Menu new-menu not defined”. But I defined it by

register_nav_menu('new-menu', 'New Menu');

didn’t I? So where can be problem?

Thanks a lot for every advice.