“count(): Argument #1 ($value) must be of type Countable|array” in WordPress: what it means and how to fix it

The message, exactly

A theme counts the slides of a slider, and nothing has been saved yet. On PHP 7.4 that gives a warning and the page goes on; on PHP 8.0 the page stops:

<?php
function get_slides() {
    return null; // nothing saved yet
}

$slides = get_slides();
echo 'Slides: ', count($slides), "\n";

Slides:
Warning: count(): Parameter must be an array or an object that implements Countable in /var/www/html/wp-content/themes/old-theme/functions.php on line 7
0

Slides:
Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in /var/www/html/wp-content/themes/old-theme/functions.php:7
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/themes/old-theme/functions.php on line 7

php.net’s changelog for count(): “count() will now throw TypeError on invalid countable types passed to the value parameter.” (PHP 8.0.0), and before that, from 7.2.0: “count() will now yield a warning on invalid countable types passed to the value parameter.” (php.net)

On a WordPress site you see “There has been a critical error on this website.” and WordPress emails the administrator. The full message is in that email, or in your error log.

What “null given” or “string given” tells you

The last words say what count() received instead of a list. A single tag saved as text, for example, used to count as one; on PHP 8 it stops the page too:

<?php
$tags = 'news'; // one tag saved as text instead of a list
echo 'Tags: ', count($tags), "\n";

Tags:
Warning: count(): Parameter must be an array or an object that implements Countable in /var/www/html/wp-content/themes/old-theme/functions.php on line 3
1

Tags:
Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, string given in /var/www/html/wp-content/themes/old-theme/functions.php:3
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/themes/old-theme/functions.php on line 3

null given usually means a value that was never saved; bool given often means false, which many WordPress functions return when they find nothing.

Is it the plugin, the theme, or WordPress?

Read the path in the message: the folder after wp-content/plugins/ or wp-content/themes/ is the culprit, here a theme called old-theme. If the path is in wp-includes/ or wp-admin/, look further down the message (the stack trace) for the first wp-content/ path: WordPress was only passing along a value a plugin gave it.

Fix it, in order

Why no check finds it beforehand

The code count($slides) is correct as long as $slides is a list. Whether it is depends on what is saved on your site, so a check that reads code can’t know. CompatNav doesn’t report it; its scan of the examples on this page finds nothing. This is one of the problems that only your error log shows: the WordPress debug log explains how to keep one without showing errors to visitors, and checking it for a few days after a PHP switch catches it early.