RainbowThemes Responsive Menu

<?php

class Mt_Walker_Nav_Menu_Dropdown extends Walker_Nav_Menu {

    private $closed = true;

    function start_lvl(&$output, $depth) {
        $indent = str_repeat("\t", $depth); // don't output children opening tag (`<ul>`)
    }

    function end_lvl(&$output, $depth) {
        $indent = str_repeat("\t", $depth); // don't output children closing tag
    }

    function start_el(&$output, $item, $depth, $args) {

        if( !$this->closed ) {
            $output .= "</option>\n";
            $this->closed = true;
        }

        // add spacing to the title based on the depth
        $item->title = str_repeat("&nbsp;", $depth * 4) . $item->title;

        parent::start_el(&$output, $item, $depth, $args);

        // no point redefining this method too, we just replace the li tag...
        $output = str_replace('<li', '<option', $output);
        $output = str_replace('</a>', '', $output);
        $output = preg_replace('/><a href="(.*?)">/', ' value="\\1">', $output);
        $output = preg_replace('/class="(.*?) current-menu-item(.*?)"/', 'selected="selected"', $output);
        $output = preg_replace('/ class="(.*?)"/', '', $output);

        $this->closed = false;
    }

    function end_el(&$output, $item, $depth) {
    }

}



// usage

wp_nav_menu(array(
    'container_class' => 'mt-responsive-menu',
    'theme_location' => 'primary',
    'walker' => new Mt_Walker_Nav_Menu_Dropdown(),
    'items_wrap' => '<select><option value="#">'. __('Navigate to...', 'mt') .'</option>%3$s</option></select>'
));