使用PHP在Wordpress中访问图像ALT标签以获取图像？

Question

如何访问图像ALT标签并将其添加到以下wordpress模板中的字符串中？

<?php
$id = get_the_id();
$thumb_id = get_post_thumbnail_id($id);
$med_file = wp_get_attachment_image_src($thumb_id, 'medium');
$full_file = wp_get_attachment_image_src($thumb_id, 'full');
$string.="<div data-med='{$med_file[0]}' data-full='{$full_file[0]}' data- 
gallery_number='{$counter}' class='{$grid_class} col-12-med col-12-small 
single_service col-3 flex'>";
$string.="<img class='col-12' src='{$med_file[0]}' />";
$string.="</div>";