updated Walker_Nav_Menu_Dropdown class for strict standards

navigation.php

<?php

/**
 * source: http://www.billerickson.net/code/wordpress-menu-as-select-dropdown/
 * Use this class to set up a nav menu to use as a dropdown menu.
 */

class Walker_Nav_Menu_Dropdown extends Walker_Nav_Menu{

	function start_lvl( &$output, $depth = 0, $args = array() ){
		$indent = str_repeat("\t", $depth); // don't output children opening tag (`<ul>`)
	}

	function end_lvl( &$output, $depth = 0, $args = array() ){
		$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 = 0, $args = array(), $id = 0 ) {
 		$url = '#' !== $item->url ? $item->url : '';
 		$output .= '<option value="' . $url . '">' . $item->title;
	}

	function end_el( &$output, $item, $depth = 0, $args = array() ){
		$output .= "</option>\n"; // replace closing </li> with the option tag
	}
}