有没有办法根据我定义的自定义字段（last\\u name）按字母顺序对这些帖子进行排序？

<?php
$page_id = get_the_ID();

$args = array(\'sort_order\'=>\'DESC\', \'post_type\'=>\'page\', \'child_of\'=>$page_id, \'post_status\'=>\'publish\');
$posts_array = get_pages($args);


foreach ($posts_array as $post){
    setup_postdata($post);
    ?>
    <li>
        (post content here)
    </li>
    <?php
}
?>