alt文本作为post meta存储在附件中，可以使用get_post_meta() 与…结合get_post_thumbnail_id():

<?php echo get_post_meta( get_post_thumbnail_id(), \'_wp_attachment_image_alt\', true ); ?>

<?php the_post_thumbnail( \'post-thumbnail\', [\'class\' => \'vehicle-photo\'] ); ?>

请尝试以下代码：

<?php $image_id = get_post_thumbnail_id(get_the_ID());
$alt_text = get_post_meta($image_id , \'_wp_attachment_image_alt\', true);?>//Variable name updated

<img class="vehicle-photo" src="<?php echo the_post_thumbnail_url() ?>" alt="<?php echo $alt_text ;?>" />