Upgraded php & wordpress but theme broke

Question

I just upgraded wordpress from v3.3 to v6.6 and PHP from 7.2 to 8.2. Once upgraded my wordpress theme just break and I have no idea how to fix this since it worked on the older version. The error shows up saying:

Fatal error: Uncaught Error: Undefined constant "thumbnail" in /var/www/noname.com/htdocs/wp-content/themes/noname/loop.php:17 Stack trace: #0 /var/www/noname.com/htdocs/wp-includes/template.php(812): require() #1 /var/www/noname.com/htdocs/wp-includes/template.php(745): load_template() #2 /var/www/noname.com/htdocs/wp-includes/general-template.php(206): locate_template() #3 /var/www/noname.com/htdocs/wp-content/themes/noname/index.php(26): get_template_part() #4 /var/www/noname.com/htdocs/wp-includes/template-loader.php(106): include('...') #5 /var/www/noname.com/htdocs/wp-blog-header.php(19): require_once('...') #6 /var/www/noname.com/htdocs/index.php(17): require('...') #7 {main} thrown in /var/www/noname.com/htdocs/wp-content/themes/noname/loop.php on line 17

loop.php

<?php
/*
Template Name: Main
*/
?>
<?php
 get_header();
?>
<h1 class="recent-heading">Latest CG Sets</h1>
<div class="recentbox">
<ul>
<?php query_posts('category_name=cg&showposts=21'); ?>
<?php while (have_posts()) : the_post(); ?>
<li> <div class="recent-title"><h2><a href="<?php the_permalink(); ?>">
          <?php the_title(); ?></a>  </h2></div>
          
<div class="recent-image"><a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>" ><?php the_post_thumbnail(thumbnail); ?></a></div>
<div class="recent-time"><?php the_time('D, F jS, Y') ?></div></li>
        <?php endwhile; ?>

</ul>
</div>
</div>

Can anyone help me fix my theme since it no longer works on new version?

Answer 1

<?php the_post_thumbnail(thumbnail); ?> should be <?php the_post_thumbnail( "thumbnail" ); ?>. thumbnail is not a defined constant, and the updated version of PHP is no longer converting undefined constants into strings for you.

It appears that PHP changed the "undefined constants" message from a notice to a fatal error sometime around version 8.