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<?php
/**
 * 検索結果URLの変更
 * URL: ドメイン/search/?s=検索ワード
 * @param $link
 * @return mixed|string
 */
function custom_search_link($link) {
    $link = str_replace('/search/','/search/?s=',$link);
    $link = rtrim($link, '/');
    return $link;
}
add_filter('search_link','custom_search_link');