首先，您必须提供下拉列表名称，以便：

<?php $media = array(
 \'name\'               => \'subcat\',
 \'hierarchical\'       => 1,
 \'parent\'             => get_cat_id(\'Media\'),
 \'show_option_none\'   => (\'All Media\'),
 \'hide_empty\'   => 0  ); 
?>

<form method="get" id="searchform" action="<?php bloginfo(\'url\'); ?>/">
  <div>
    <input type="text" value="<?php the_search_query(); ?>" name="s" id="s" />
    <?php wp_dropdown_categories(\'name=maincat&show_option_none=All Category\'); ?>
    <?php wp_dropdown_categories($media); ?>
    <input type="hidden" id="my_search" name="my_search" value="c_search" />
    <input type="submit" id="searchsubmit" value="search" />
  </div>
</form>

// Define search filter
function search_filter( $query ) {
    // only modify your custom search query.
    if ( $query->is_search &&  $_post[\'my_search\'] == "c_search") {
        $args = array(
                \'relation\' => \'AND\',
            array(
                \'taxonomy\' => \'category\',
                \'field\' => \'id\',
                \'terms\' => array( $_post[\'maincat\']),
                \'operator\' => \'IN\'
            ),
            array(
                \'taxonomy\' => \'category\',
                \'field\' => \'id\',
                \'terms\' => array( $_post[\'subcat\']),
                \'operator\' => \'IN\'
            )
        );
        $query->set( \'tax_query\', $args);
    }
    return $query;
}

// The hook needed to search_filter
add_filter( \'the_search_query\',\'search_filter\');