“There has been a critical error on this website” after a PHP update: find the cause and fix it

First, get your site back

WordPress shows “There has been a critical error on this website.” when PHP stops with a fatal error while building a page. Since WordPress 5.2 it catches these errors, shows an error message instead of a blank page, and emails the site administrator (make.wordpress.org).

If the message appeared right after a PHP update, undo the update first and investigate afterwards:

Can’t change it yourself? Ask your host’s support to switch your site back; it’s a routine request. Going back is a pause, not a solution: PHP versions reach their end of life (see PHP 8.2 reaches end of life on 31 December 2026), so the cause has to be fixed.

Find the plugin or theme that stopped

The email WordPress sends you

When the error happens, WordPress emails the site’s administration email address. The subject contains “Your Site is Experiencing a Technical Issue”, and the email names the culprit: “In this case, WordPress caught an error with one of your plugins, …” or “… with your theme, …”. It also contains the technical error message and a link to recovery mode (WordPress 7.1.2 source).

On the admin and login pages of a single site, the error message itself reminds you: “Please check your site admin email inbox for instructions.”

Recovery mode

The link in the email logs you in while the plugin or theme that caused the error is paused. The admin area tells you which extensions are paused and what the errors are, so you can deactivate the plugin or switch the theme. You leave recovery mode with the button in the admin bar (make.wordpress.org).

No email? Find the error message

Your host usually keeps a PHP error log; its control panel or support can show it to you. If you aren’t comfortable editing your site’s files, this is the way to go: ask your host’s support for the PHP error log from the time the error appeared.

WordPress can also write errors to a log file. For that you edit wp-config.php, usually in the main folder of your WordPress site, with your host’s File Manager or an FTP program: set WP_DEBUG and WP_DEBUG_LOG to true and WP_DEBUG_DISPLAY to false. The WordPress documentation says these debug tools are “not recommended” on live sites, and by default the log is written to wp-content/debug.log (developer.wordpress.org). If you use it on a live site, point WP_DEBUG_LOG to a file outside the public web folder, as the documentation allows, and switch it off again when you’re done.

Reading the error message

A fatal error message tells you what went wrong, and where. This is the real message PHP 8.0 gives for a plugin that still uses create_function():

Fatal error: Uncaught Error: 1Call to undefined function create_function() in /var/www/html/2wp-content/plugins/old-slider/old-slider.php:3
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/plugins/old-slider/3old-slider.php on line 3

Why a PHP update causes fatal errors

Each PHP version removes things that were announced as deprecated earlier, and changes how some code behaves. PHP 8.0, for example, removed several features that PHP 7 had already deprecated. Old plugin code runs into such changes in three ways.

A function that no longer exists

create_function() was deprecated in PHP 7.2 “given the security issues of this function (being a thin wrapper around eval())” (php.net). PHP 8.0 removed it, along with each(): “create_function() has been removed. Anonymous functions may be used instead.” and “each() has been removed. foreach or ArrayIterator should be used instead.” (php.net).

On PHP 7.4 the code still runs and only logs a deprecation notice. On PHP 8.0 and later the call is a fatal error.

<?php
$slides = [['title' => 'Team', 'order' => 2], ['title' => 'Welcome', 'order' => 1]];
$by_order = create_function('$a, $b', 'return $a["order"] - $b["order"];');
usort($slides, $by_order);
echo 'First slide: ', $slides[0]['title'], "\n";

Deprecated: Function create_function() is deprecated in /var/www/html/wp-content/plugins/old-slider/old-slider.php on line 3
First slide: Welcome

Fatal error: Uncaught Error: Call to undefined function create_function() in /var/www/html/wp-content/plugins/old-slider/old-slider.php:3
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/plugins/old-slider/old-slider.php on line 3

<?php
$slides = [['title' => 'Team', 'order' => 2], ['title' => 'Welcome', 'order' => 1]];
$by_order = function ($a, $b) {
    return $a['order'] - $b['order'];
};
usort($slides, $by_order);
echo 'First slide: ', $slides[0]['title'], "\n";

First slide: Welcome

First slide: Welcome

<?php
$options = ['color' => 'blue', 'layout' => 'wide'];
while (list($key, $value) = each($options)) {
    echo $key, ' = ', $value, "\n";
}

Deprecated: The each() function is deprecated. This message will be suppressed on further calls in /var/www/html/wp-content/themes/old-theme/functions.php on line 3
color = blue
layout = wide

Fatal error: Uncaught Error: Call to undefined function each() 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

<?php
$options = ['color' => 'blue', 'layout' => 'wide'];
foreach ($options as $key => $value) {
    echo $key, ' = ', $value, "\n";
}

color = blue
layout = wide

Code the new version refuses to compile

Some code isn’t even loaded any more. PHP 8.0: “Nested ternaries now require explicit parentheses.” (php.net) A line like $a ? 'x' : $b ? 'y' : 'z' without brackets stops the whole file from compiling, so every page that loads it fails.

<?php
$count = 0;
$label = $count == 0 ? 'no photos' : $count == 1 ? 'one photo' : 'photos';
echo $label, "\n";

Deprecated: Unparenthesized `a ? b : c ? d : e` is deprecated. Use either `(a ? b : c) ? d : e` or `a ? b : (c ? d : e)` in /var/www/html/wp-content/plugins/old-gallery/old-gallery.php on line 3
one photo

Fatal error: Unparenthesized `a ? b : c ? d : e` is not supported. Use either `(a ? b : c) ? d : e` or `a ? b : (c ? d : e)` in /var/www/html/wp-content/plugins/old-gallery/old-gallery.php on line 3

<?php
$count = 0;
$label = $count == 0 ? 'no photos' : ($count == 1 ? 'one photo' : 'photos');
echo $label, "\n";

no photos

Code that silently changes meaning

The trickiest case produces no error where the problem is. Very old PHP code creates objects with a method that has the same name as its class, instead of __construct(). PHP 8.0: “Methods with the same name as the class are no longer interpreted as constructors.” (php.net)

On PHP 8.0 that method simply isn’t run when the object is created. There is no error at that line: the object stays uninitialised, and the page fails later, wherever the missing data is used. The error message then points to that later line, not to the real cause.

<?php
class Slider {
    private $slides;

    public function Slider() {
        $this->slides = ['welcome.jpg', 'team.jpg'];
    }

    public function count_slides() {
        return count($this->slides);
    }
}

$slider = new Slider();
echo 'Slides: ', $slider->count_slides(), "\n";

Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; Slider has a deprecated constructor in /var/www/html/wp-content/plugins/old-slider/includes/class-slider.php on line 2
Slides: 2

Slides:
Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in /var/www/html/wp-content/plugins/old-slider/includes/class-slider.php:10
Stack trace:
#0 /var/www/html/wp-content/plugins/old-slider/includes/class-slider.php(15): Slider->count_slides()
#1 {main}
  thrown in /var/www/html/wp-content/plugins/old-slider/includes/class-slider.php on line 10

<?php
class Slider {
    private $slides;

    public function __construct() {
        $this->slides = ['welcome.jpg', 'team.jpg'];
    }

    public function count_slides() {
        return count($this->slides);
    }
}

$slider = new Slider();
echo 'Slides: ', $slider->count_slides(), "\n";

Slides: 2

Reading the code, rather than waiting for the error, finds the cause directly. This is how a check reports this plugin before the upgrade:

Fix it

• Update the plugin or theme (Dashboard → Updates). If an update is available, install it first: it may already contain the fix.
• Already on the latest version, or no updates for years? Contact its developer, or replace it with a maintained alternative.
• Code written for your site? Give the error message to whoever maintains it.
• Don’t edit the plugin’s or theme’s files yourself: your changes are lost with the next update.

Not ready yet: what to do explains each option in more detail.

Upgrade PHP again, safely

What a scan can’t catch

A check that reads code finds removed functions, code PHP refuses to compile, and patterns like old-style constructors before you switch. It can’t see problems that depend on the values a plugin works with while it runs. For example, PHP 8.0 turned “a number of warnings” into errors, such as arithmetic on a value that isn’t a number (php.net): whether that happens depends on the data. That’s why the last step, checking the site after switching, always matters.

For the PHP 8.5 changes specifically, see WordPress and PHP 8.5: what breaks, what only shows notices.