Wordpress - Determine if a menu item has children - php

Question

I've searched for hours and cannot come up with what I thought would be a simple thing. I am building my very first Wordpress site locally. I'm making a function for my functions.php file based on this howto but I need it to have submenus. I have created a custom menu inside Wordpress with 2 levels.
I need to add HTML code (not a CSS class) to an item if it is a parent item.
Bonus points if you can tell me how people know what attributes/properties are available to a menu item (such as title and url) - I can't find a reference to that anywhere!
Here is my current code, which is currently displaying all menu items as top-level items.
function clean_custom_main_menu() {
$menu_name = 'main-menu'; // specify custom menu slug
$menu_list = '<ul id="menu">' ."\n";
if ($menu_items = wp_get_nav_menu_items($menu_name)) {
$count = 0;
$submenu = false;
foreach ((array) $menu_items as $key => $menu_item) {
$title = $menu_item->title;
$url = $menu_item->url;
$classes = $menu_item->classes; // does not work
$has_children = $menu_item->has_children; //does not work
$parent_id = 0;
// check if this item is a parent item with children
//if ( $menu_item->menu_item_parent && $menu_item->menu_item_parent > 0 ) {
//if(in_array('menu-item-has-children', $classes)){
if ($has_children) {
$parent_id = $menu_item->ID;
}
// if this item has a parent ID, it's a second-level item
if ($parent_id != 0 && $parent_id == $menu_item->menu_item_parent ) {
$submenu = true;
}
if (!$has_children) {
// if this item has no submenu, write top-level code
$menu_list .= "\t". '<li>'. $title .'</li>' ."\n";
}
// the "else" is not currently being hit
//else {
// $menu_list .= "\t" . '<li>' . $title . '<span class="arrow-down"></span></li>' . "\n";
// $menu_list .= '<ul class="sub">';
// // foreach (child) do children
// $menu_list .= '</ul>';
// $menu_list .= '</li>';
//}
}
} else {
$menu_list .= '<!-- no list defined -->';
}
$menu_list .= "\t". '</ul>' ."\n";
echo $menu_list;
}
Desired output:
<ul id="menu">
<li>Home</li>
<li>
About <span class="arrow-down"></span>
<ul class="sub">
<li>Our Services</li>
<li>What We Do</li>
</ul>
</li>
<li>
Top-Level Item
</li>
<li>Top-Level Item with Children <span class="arrow-down"></span>
<ul class="sub">
<li>Sub Item 1</li>
<li>Sub Item 2</li>
<li>Sub Item 3</li>
<li>Sub Item 4</li>
</ul>
</li>
<li>Contact</li>
</ul>
Please note: I do not want to use walkers. I am certain it can be done somehow since I had this working at one point, but lost everything when Wordpress decided to update the theme I made - to a totally different one from web-land, erasing all my files. D'oh!

Answer 1

It turns out that people have contributed how to build items with submenus right on the Wordpress documentation for the function I was using, wp_get_nav_menu_items(). Those got me started but they weren't exactly quite right, since I had the extra span tags and wanted to close my A tags in different places, but I was able to ultimately get things working. Here is my final code based on those comments:
function clean_custom_main_menu() {
$menu_name = 'main-menu'; // specify custom menu slug
$menu_list = '<ul id="menu">' ."\n";
if ($menu_items = wp_get_nav_menu_items($menu_name)) {
$count = 0;
$submenu = false;
$parent_id = 0;
$previous_item_has_submenu = false;
foreach ((array) $menu_items as $key => $menu_item) {
$title = $menu_item->title;
$url = $menu_item->url;
// check if it's a top-level item
if ($menu_item->menu_item_parent == 0) {
$parent_id = $menu_item->ID;
// write the item but DON'T close the A or LI until we know if it has children!
$menu_list .= "\t". '<li><a href="'. $url .'">'. $title;
}
// if this item has a (nonzero) parent ID, it's a second-level (child) item
else {
if ( !$submenu ) { // first item
// add the dropdown arrow to the parent
$menu_list .= '<span class="arrow-down"></span></a>' . "\n";
// start the child list
$submenu = true;
$previous_item_has_submenu = true;
$menu_list .= "\t\t" . '<ul class="sub">' ."\n";
}
$menu_list .= "\t\t\t" . '<li>';
$menu_list .= ''.$title.'';
$menu_list .= '</li>' ."\n";
// if it's the last child, close the submenu code
if ( $menu_items[ $count + 1 ]->menu_item_parent != $parent_id && $submenu ){
$menu_list .= "\t\t" . '</ul></li>' ."\n";
$submenu = false;
}
}
// close the parent (top-level) item
if (empty($menu_items[$count + 1]) || $menu_items[ $count + 1 ]->menu_item_parent != $parent_id )
{
if ($previous_item_has_submenu)
{
// the a link and list item were already closed
$previous_item_has_submenu = false; //reset
}
else {
// close a link and list item
$menu_list .= "\t" . '</a></li>' . "\n";
}
}
$count++;
}
} else {
$menu_list .= '<!-- no list defined -->';
}
$menu_list .= "\t". '</ul>' ."\n";
echo $menu_list;
}

Answer 2

API
Try doing var_dump( $menu_items ) and you'll see that the menu items are posts of post_type "nav_menu_item" so they have the same api as post.
Count Children
https://wordpress.stackexchange.com/questions/9374/get-the-post-children-count-of-a-post shows a way of accessing child post count using $wpdb. You could do this with get_posts by setting the parent_id to the parent item and post_type to nav_menu_item and then count the results.
Classes
The post object has a classes array. Since this wasn't working for you, I'd double check how you are trying to access/use that array and if that doesn't help, try investigating the walker functions for help with your custom implementation.