A slightly improved 'Add Nav Extras' code for adding Search in Navigation. http://sridharkatakam.com/slightly-improved-add-nav-extras-code-adding-search-navigation/

functions.php

<?php
//* Do NOT include the opening php tag

add_filter( 'wp_nav_menu_items', 'theme_menu_extras', 10, 2 );
/**
 * Filter menu items, appending a search form.
 *
 * @param string   $menu HTML string of list items.
 * @param stdClass $args Menu arguments.
 *
 * @return string Amended HTML string of list items.
 */
function theme_menu_extras( $menu, $args ) {

	//* Change 'primary' to 'secondary' to add search form to the secondary navigation menu
	if ( 'primary' !== $args->theme_location )
		return $menu;

	$menu  .= '<li class="right search">' . get_search_form( false ) . '</li>';

	return $menu;

}

gistfile1.php

ob_start();
get_search_form();
$search = ob_get_clean();
$menu  .= '<li class="right search">' . $search . '</li>';

gistfile2.php

$menu  .= '<li class="right search">' . get_search_form( false ) . '</li>';