get_post_meta remains empty while looping all menu items and using the ID

Question

I am trying to create some sort of custom menu. Where I list all titles of my pages in the main menu and display the value of a meta box of each of these pages. For some reason the get_post_meta remains empty for all items. I fail to see why.

<?php
 $array_menu = wp_get_nav_menu_items('header-menu');
    $menu = array();
    foreach ($array_menu as $m) {
        if (empty($m->menu_item_parent)) {
             ?>
             <h4><a href="<?php echo $m->url;?>"><?php echo $m->title;?></a></h4>
             <p>
                <?php echo get_post_meta($m->ID, 'meta_box_subtitle', true); ?>
             </p>
<?php       
        }
    }
?>

Answer 1

You are sending the ID of the menu item not the ID of the page try using url_to_postid() like this:

<?php
 $array_menu = wp_get_nav_menu_items('header-menu');
    $menu = array();
    foreach ($array_menu as $m) {
        if (empty($m->menu_item_parent)) {
             ?>
             <h4><a href="<?php echo $m->url;?>"><?php echo $m->title;?></a></h4>
             <p>
                <?php global $wp_query;
                 $pageID = url_to_postid( $m->url );  
                 echo get_post_meta($pageID, 'meta_box_subtitle', true); 
                ?>
             </p>
<?php       
        }
    }
?>