好的，所以我找到了解决我的问题的方法。下面是粘贴的代码；对某人可能有帮助。

<form method=post>
    <div class="header-right">
      <?php
      $posts = get_pages(
        array(
            'post_status' => 'publish',
        )
      );
      
      ?>
      <?php
      if(empty($_POST['page_for_logged_in'])) {
          
        } else {
          $myvalue=$_POST['page_for_logged_in'];
          update_option('page_for_logged_in', $myvalue, $autoload = 'yes');
        }
        ?>
      <select name="page_for_logged_in" id="page_for_logged_in">
        <option value="" disabled selected>选择一个</option>
        <?php
        foreach ( $posts as $page ) {
            ?>
        <option value="<?php echo esc_attr( $page->post_title ); ?>" <?php echo ( get_option('page_for_logged_in') == $page->post_title ? 'selected' : '' ); ?>><?php echo esc_html( $page->post_title ); ?></option>
            <?php
        }
        ?>
      </select>

    <?php submit_button(); ?>

    </p>
    </br>
    </br>
    </form>