Add value to new attribute inside Wordpress menu <li> items

Question

I have this code in my functions.php to add new attribute data-content="" to all my <li> items in the wordpress menu. The value of each data-content has to be different: I would like that each value corresponds to the inside text (in slug format) of the <a> tag.

function add_attribute( $items, $args ) {
    $dom = new DOMDocument();
    $dom->loadHTML($items);
    $find = $dom->getElementsByTagName('li');

    foreach ($find as $item ) :
        $item->setAttribute('data-content','');

    endforeach;

    return $dom->saveHTML();

}
add_filter('wp_nav_menu_items', 'add_attribute', 10, 2);

Here's how the final result (in html) should be:

<ul>
  <li class="menu-item" data-content="about">
    <a href="#">About</a>
  </li>

  <li class="menu-item" data-content="my-services">
    <a href="#">My services</a>
  </li>

  <li class="menu-item" data-content="contact-me">
    <a href="#">Contact me</a>
  </li>
</ul>

For the moment the PHP code just creates the attribute but does not add any value inside data-content. I'm struggling to find the way how do it... Any help is much appreciated, thanks!

Upadate: I tried the PHP function nav_menu_link_attributes. Unfortunately it doesn't work on <li> elements. The function add a new attribute and its value on each <a> elements of the menu.

add_filter( 'nav_menu_link_attributes', 'cfw_add_data_atts_to_nav', 10, 4 );
function cfw_add_data_atts_to_nav( $atts, $item, $args )
{
    $atts['data-content'] = sanitize_title($item->title);
    return $atts;
}

Answer 1

I already had this need, I found this code somewhere in the web. up to you to customize it with your needs. I tested it on the Twenty Nineteen theme:

Put this in functions.php :

class themeslug_walker_nav_menu extends Walker_Nav_Menu {

 // add classes to ul sub-menus
function start_lvl(&$output, $depth) {
    // depth dependent classes
    $indent = ( $depth > 0 ? str_repeat("\t", $depth) : '' ); // code indent
    $display_depth = ( $depth + 1); // because it counts the first submenu as 0
    $classes = array(
        'sub-menu',
        ( $display_depth % 2 ? 'menu-odd' : 'menu-even' ),
        ( $display_depth >= 2 ? 'sub-sub-menu' : '' ),
        'menu-depth-' . $display_depth
    );
    $class_names = implode(' ', $classes);

    // build html
    $output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
}

// add main/sub classes to li's and links
function start_el(&$output, $item, $depth, $args) {
    global $wp_query;
    $indent = ( $depth > 0 ? str_repeat("\t", $depth) : '' ); // code indent
    // depth dependent classes
    $depth_classes = array(
        ( $depth == 0 ? 'main-menu-item' : 'sub-menu-item' ),
        ( $depth >= 2 ? 'sub-sub-menu-item' : '' ),
        ( $depth % 2 ? 'menu-item-odd' : 'menu-item-even' ),
        'menu-item-depth-' . $depth
    );
    $depth_class_names = esc_attr(implode(' ', $depth_classes));

    // passed classes
    $classes = empty($item->classes) ? array() : (array) $item->classes;
    $class_names = esc_attr(implode(' ', apply_filters('nav_menu_css_class', array_filter($classes), $item)));

    // build html
    $output .= $indent . '<li data-content="'.$item->title.'"  id="nav-menu-item-' . $item->ID . '" class="' . $depth_class_names . ' ' . $class_names . '">';

    // link attributes
    $attributes = !empty($item->attr_title) ? ' title="' . esc_attr($item->attr_title) . '"' : '';
    $attributes .=!empty($item->target) ? ' target="' . esc_attr($item->target) . '"' : '';
    $attributes .=!empty($item->xfn) ? ' rel="' . esc_attr($item->xfn) . '"' : '';
    $attributes .=!empty($item->url) ? ' href="' . esc_attr($item->url) . '"' : '';
    $attributes .= ' class="menu-link ' . ( $depth > 0 ? 'sub-menu-link' : 'main-menu-link' ) . '"';

    $item_output = sprintf('%1$s<a%2$s>%3$s%4$s%5$s</a>%6$s', $args->before, $attributes, $args->link_before, apply_filters('the_title', $item->title, $item->ID), $args->link_after, $args->after
    );

    // build html
    $output .= apply_filters('walker_nav_menu_start_el', $item_output, $item, $depth, $args);
    }

}

And in header.php :

wp_nav_menu( array(
   'theme_location'    => 'menu-1',
   'container'     => 'div',
   'container_id'      => 'top-navigation-primary',
   'conatiner_class'   => 'top-navigation',
   'menu_class'        => 'menu main-menu menu-depth-0 menu-even', 
   'echo'          => true,
   'items_wrap'        => '<ul id="%1$s" class="%2$s">%3$s</ul>',
   'depth'         => 10, 
   'walker'        => new themeslug_walker_nav_menu
) ); // thanks nick

So adapt the code to your needs. good luck