Multi level menu with PHP/MySQL

I'm trying to create dynamic multi level menus fetching the data from a MySQL DB, using PHP. I've managed to order the menu items in a php array with this format:

-----------------------
Array
(
[1] => Array
    (
        [id] => 1
        [ubicacion] => top_a
        [nivel] => 1
        [parent_id] => 
        [tipo] => link
        [link] => http://www.google.com
        [titulo] => Google
        [alias] => google_es
        [children] => Array
            (
                [3] => Array
                    (
                        [id] => 3
                        [ubicacion] => top_a
                        [nivel] => 2
                        [parent_id] => 1
                        [tipo] => link
                        [link] => http://www.gmail.com
                        [titulo] => Gmail
                        [alias] => gmail
                        [children] => Array
                            (
                                [4] => Array
                                    (
                                        [id] => 4
                                        [ubicacion] => top_a
                                        [nivel] => 3
                                        [parent_id] => 3
                                        [tipo] => link
                                        [link] => www.inbox.gmail.com
                                        [titulo] => Inbox
                                        [alias] => inbox_gmail
                                    )

                            )

                    )

            )
    )

[2] => Array
    (
        [id] => 2
        [ubicacion] => top_a
        [nivel] => 1
        [parent_id] => 
        [tipo] => link
        [link] => http://www.yahoo.com
        [titulo] => Yahoo
        [alias] => yahoo
    )
)
-----------------------

The problem is that I can't figure out how to output this array as HTML markup in a way that will work with n levels. I can do it with a fixed number of levels like this:

foreach($menu_array as $menu) {
 echo "<li><a href='{$menu['link']}'>{$menu['titulo']}</a>";
 if (array_key_exists('children',$menu)) {
    echo "<ul>";
    foreach ($menu['children'] as $child_menu) {
        echo "<li><a href='{$child_menu['link']}'>{$child_menu['titulo']}</a>";
        if (array_key_exists('children',$child_menu)) {
            echo "<ul>";
            foreach ($child_menu['children'] as $child2_menu) {
                echo "<li><a href='{$child2_menu['link']}'>{$child2_menu['titulo']}</a>";
            }
            echo "</ul>";
        }
    }
    echo "</ul>";
}
echo "</li>";
}

But this only works for 3 levels, and I know there should be a way to solve this issue, I know I'm not the first one facing a problem with HTML output of a multidimensional array.

You can just use a little bit of recursion to get you to more levels.

function echo_menu($menu_array) {
    //go through each top level menu item
    foreach($menu_array as $menu) {
        echo "<li><a href='{$menu['link']}'>{$menu['titulo']}</a>";
        //see if this menu has children
        if(array_key_exists('children', $menu)) {
            echo '<ul>';
            //echo the child menu
            echo_menu($menu['children']);
            echo '</ul>';
        }
        echo '</li>';
    }
}

echo '<ul>';
echo_menu($menu_array);
echo '</ul>';

This will work for any number of child levels you'd like.