WordPress and PHP 8.1: what changes for plugins and themes

Is PHP 8.1 still a good choice?

No. php.net lists PHP 8.1 among the unsupported branches, with an end of life on 31 December 2025 (php.net). Every version that is still supported builds on it, though: a site moving from PHP 7.4 or 8.0 to 8.3 or later meets the PHP 8.1 changes on the way, so they are worth knowing.

The WordPress core team’s compatibility page lists WordPress 5.9 and later as compatible with PHP 8.1. The changes below concern the code of your plugins and theme.

The short version

The PHP 8.1 changes you’re most likely to meet in plugins and themes. php.net lists the full set of deprecated features and backward incompatible changes.

The one that stops code: $GLOBALS

php.net: “write access to the entire $GLOBALS array is no longer supported.” (php.net) Writing one entry, $GLOBALS['name'] = …, still works. Replacing the array as a whole is now a fatal error, and PHP refuses the whole file:

<?php
$GLOBALS = ['site_name' => 'My shop'];
echo 'Saved', "\n";

Saved

Fatal error: $GLOBALS can only be modified using the $GLOBALS[$name] = $value syntax in /var/www/html/wp-content/plugins/old-settings/old-settings.php on line 2

It is rare in plugins, but when it happens the page stops, so it is the one to fix before switching.

strftime() and other date functions

strftime() formatted dates by the server’s language settings. php.net: “strftime() and gmstrftime() have been deprecated. Use date() instead (for locale-independent formatting), or IntlDateFormatter::format() (for locale-dependent formatting).” (php.net)

<?php
$timestamp = 1798675200; // 31 December 2026, 00:00 UTC
echo strftime('%Y-%m-%d', $timestamp), "\n";

2026-12-31

Deprecated: Function strftime() is deprecated in /var/www/html/wp-content/plugins/event-dates/event-dates.php on line 3
2026-12-31

FILTER_SANITIZE_STRING, null and return types

Three PHP 8.1 notices are common enough to have their own guides:

• “Constant FILTER_SANITIZE_STRING is deprecated”: there is no exact replacement, so the developer chooses one.
• “Passing null to parameter … is deprecated”: depends on values, so only your error log shows it.
• “Return type of … should either be compatible …”: covered in moving from PHP 7.4 to PHP 8, with the attribute that silences it and the real fix.

The Serializable interface

Plugins that store objects, for example in a session or a cache, sometimes implement Serializable. PHP 8.1 asks for the newer methods instead:

<?php
class Cart implements Serializable {
    private $items = [];

    public function serialize() {
        return serialize($this->items);
    }

    public function unserialize($data) {
        $this->items = unserialize($data);
    }
}

echo 'Loaded', "\n";

Loaded

Deprecated: Cart implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /var/www/html/wp-content/plugins/cart-session/cart.php on line 2
Loaded

The message itself names the fix: add __serialize() and __unserialize(), alongside the old methods if the plugin still supports PHP older than 7.4.

Notices that depend on values

php.net calls it autovivification from false: “Autovivification is prohibited from scalar values, false however was an exception. This is now deprecated.” (php.net) In WordPress it happens when code reads a saved setting that doesn’t exist yet (WordPress returns false) and then adds entries to it as if it were a list:

<?php
function get_saved_counts() {
    return false; // like get_option() when the option doesn't exist yet
}

$counts = get_saved_counts();
$counts['home'] = 1;
echo $counts['home'], "\n";

1

Deprecated: Automatic conversion of false to array is deprecated in /var/www/html/wp-content/plugins/visit-counter/visit-counter.php on line 7
1

A check that reads code sees $counts['home'] = 1, not that $counts is false on your site at that moment. CompatNav doesn’t report it for that reason; your error log does. The WordPress debug log shows how to keep one without showing messages to visitors.

How to check your site