wp-nav-menu-select-element.php

<?php

wp_nav_menu( array(
    'theme_location' => 'mobile',
    'walker'         => new Walker_Nav_Menu_Dropdown(),
    'items_wrap'     => '<div class="mobile-menu"><form><select onchange="if (this.value) window.location.href=this.value">%3$s</select></form></div>',
) );	

class Walker_Nav_Menu_Dropdown extends Walker_Nav_Menu {
	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
	}

	/**
	* Start the element output.
	*
	* @param  string $output Passed by reference. Used to append additional content.
	* @param  object $item   Menu item data object.
	* @param  int $depth     Depth of menu item. May be used for padding.
	* @param  array $args    Additional strings.
	* @return void
	*/
	function start_el(&$output, $item, $depth, $args) {
 		$url = '#' !== $item->url ? $item->url : '';
 		$output .= '<option value="' . $url . '">' . $item->title;
	}	

	function end_el(&$output, $item, $depth){
		$output .= "</option>\n"; // replace closing </li> with the option tag
	}
}