WPK_Nav_Walker_Select

<?php
/*
 *  Walker Class for Output Select Element from WordPress Menus
 *
 * */
class WPK_Select_Walker extends Walker_Nav_Menu {

    // add main/sub classes to li's and links
    function start_el( &$output, $item, $depth, $args ) {

        $label  = ! empty( $item->title ) ? esc_attr( $item->title ) : '';
        // build html
        $output .= '<option value="'.esc_attr($item->url).'">'.$label;
    }

    function end_el( &$output, $item, $depth, $args ) {
        $output .= '</opiton>';
    }
}

/*

Example:

$args = array(
    'theme_location'  => 'my_location',
    'menu'            => '',
    'container'       => false,
    'fallback_cb'     => false,
    'items_wrap'      => '<select id="%1$s" class="%2$s">%3$s</select>', // Important
    'depth'           => 1,
    'walker'          => new WPK_Select_Walker()
);

wp_nav_menu($args);

*/