Add Search Bar in Header

<?php

add_filter('wp_nav_menu_items','add_search_box_to_menu', 10, 2);
function add_search_box_to_menu( $items, $args ) {
    if( $args->theme_location == 'primary' )
        return $items.get_search_form();

    return $items;
}

// ή, εναλλακτικά

add_filter('wp_nav_menu_items','add_search_box_to_menu', 10, 2);
function add_search_box_to_menu( $items, $args ) {
    if( $args->theme_location == 'primary' )
        return $items."<li class='menu-header-search'><form action='http://example.com/' id='searchform' method='get'><input type='text' name='s' id='s' placeholder='Search'></form></li>";

    return $items;
}

?>