我想在我的Wordpress菜单中加入一个高级自定义字段

Here's my code now:

$menu_icon = get_field('menu_icon');

if($menu_icon) {
    $m_icon = '<img src="'.the_field('menu_icon').'">';
} else {
    $m_icon = "";
};

$personal = array(
    'theme_location'  => 'personal',
    'menu'            => 'Personal Menu',
    'before'          => '$m_icon',
);

wp_nav_menu( $personal );

And here's what it spits out:

<ul id="menu-personal-menu" class="menu">
    <li class="menu-item>
        $m_icon <a href="#">Link</a>
        <ul class="sub-menu">
            <li>$m_icon <a href="#1">SubLink 1</a></li>
            <li>$m_icon <a href="#2">SubLink 2</a></li>
            <li>$m_icon <a href="#3">SubLink 3</a></li>
        </ul>
    </li>
</ul>

I'd like to have it produce something like this:

<ul id="menu-personal-menu" class="menu">
    <li class="menu-item>
        <a href="#">Link</a>
        <ul class="sub-menu">
            <li><img src="path/sublinkimg1"> <a href="#1">SubLink 1</a></li>
            <li><img src="path/sublinkimg2"> <a href="#2">SubLink 2</a></li>
            <li><img src="path/sublinkimg3"> <a href="#3">SubLink 3</a></li>
        </ul>
    </li>
</ul>

Here's the general idea:

It's wrong for a few reasons. I know the advanced custom field: "menu_icon" probably isn't able to retrieve anything outside of a loop, and I'm not familiar enough with the wp_nav_menu() function to know whether or not it's possible to echo an image specific to the page it's displaying.

Does anyone know how I can achieve this?

One thing I notice right away is that you have extra single quotes, and php is translating the content literally. You actually want the string contained in $m_icon, and NOT the string '$m_icon' which is not a variable.

$personal = array(
    'theme_location'  => 'personal',
    'menu'            => 'Personal Menu',
    'before'          => $m_icon, //this line was wrong, single quotes means 'literally this$ $tring'
);

Edit: I took the liberty of writing a filter function for what you're trying to do. Cheers.

function insert_icons($items, $menu, $args){
    //check our menu name so we filter the right one
    if($menu->name=='Personal Menu'){
        //loop through the menu items
        foreach($items as $key => $item){
            //make sure object_id is set (this is the page or post id)
            if(isset($item->object_id) && !empty($item->object_id)){
                //grab the menu icon
                $menu_icon = get_field('menu_icon',$item->object_id);
                //make sure the previous line returned something
                if(isset($menu_icon) && !empty($menu_icon)){
                    //create our image string
                    $m_icon = '<img src="'.$menu_icon.'">';
                    //insert it into the page title
                    $item->title = $m_icon.$item->title;
                }
            }
        }
    }
    return $items;
}
add_filter('wp_get_nav_menu_items','insert_icons',NULL,3);

Not exactly what you asked for and i don't have the correct solution for your entire problem right now but i can help you with getting custom fields to work outside the loops if that helps you :)

you just have to set the page ID after your field name for it to work outside of loop like this

get_field('your-field', $page_ID);