add_theme_support( 'html5', array(
'search-form',
) );
<form role="search" method="get" id="searchform" action="<?php echo home_url( '/' ); ?>">
<input type="search" class="search" placeholder="<?php echo __( 'Поиск', '' ) ?>" value="<?php echo get_search_query() ?>" name="s" />
</form>