我找到了以下函数，可以将自定义分类下拉列表添加到搜索表单中。它可以很好地进行搜索，但不会将“selected”类添加到搜索的术语中。这是我的分类代码，名为grades:

function get_terms_grades_dropdown($taxonomies, $args)
    {
        global $search_values;

        $myterms = get_terms($taxonomies, $args);
        $output = "";
        foreach ($myterms as $term) {
            $root_url = get_bloginfo(\'url\');
            $term_taxonomy = $term->taxonomy;
            $term_slug = $term->slug;
            $term_name = $term->name;
            $value = $term->term_id;
            if ($search_values[\'grades\'] == $term->slug) {
                $selected = "selected=\'selected\'";
            } else {
                $selected = \'\';
            }
            $output .= "<option value=\'" . $value . "\' " . $selected . ">" . $term_name . "</option>";
        }

        return $output;
    }

<select name="grade" id="grade_search">
    <option value="" selected="selected">Grade Level</option>
    <?php
    $taxonomies = array(\'grades\');
    $args = array(\'orderby\'=>\'count\',\'hide_empty\'=>true);
    echo get_terms_grades_dropdown($taxonomies, $args);
    ?>
</select>

if ($search_values[\'grades\'] == $term->slug) {

非常感谢。