WordPress and PHP 8.5: what breaks, what only shows notices

Is WordPress ready for PHP 8.5?

PHP 8.5.0 was released on 20 November 2025. It gets bug fixes until 31 December 2027 and security fixes until 31 December 2029 (php.net).

WordPress 6.9 added support for PHP 8.5, labelled “beta support” until the core team retired that label in May 2026. The WordPress core team’s compatibility page records both changes and lists WordPress 6.9, 7.0 and 7.1 as compatible with PHP 8.5. That covers WordPress itself. Your site also runs plugins and a theme, and their code is what the PHP 8.5 changes affect.

The short version

The PHP 8.5 changes you’re most likely to meet in plugins and themes. php.net lists the full set of backward incompatible changes and deprecated features.

What breaks: three changes that stop the code

A fatal error stops PHP at that point: the page isn’t built, and WordPress shows “There has been a critical error on this website.” Of the PHP 8.5 changes you’re most likely to meet in plugins and themes, these three cause one. php.net lists the full set of backward incompatible changes, many of them in specific extensions.

setlocale() with 0 in files that use strict types

Some plugins ask PHP for the current language settings (the locale) by passing 0. php.net: “Passing an integer 0 as the locales argument to setlocale() is no longer supported and now throws a TypeError.” (php.net)

In our tests this happens in files that start with declare(strict_types=1). In files without it, PHP 8.5 turns 0 into '0' and the call keeps working, so only strict files are affected.

<?php
declare(strict_types=1);

$previous = setlocale(LC_NUMERIC, 0);
echo "Current numeric locale: $previous\n";

Current numeric locale: C

Fatal error: Uncaught TypeError: setlocale(): Argument #2 ($locales) must be of type array|string|null, int given in /var/www/html/wp-content/plugins/price-format/price-format.php:4
Stack trace:
#0 /var/www/html/wp-content/plugins/price-format/price-format.php(4): setlocale(4, 0)
#1 {main}
  thrown in /var/www/html/wp-content/plugins/price-format/price-format.php on line 4

<?php
declare(strict_types=1);

$previous = setlocale(LC_NUMERIC, '0');
echo "Current numeric locale: $previous\n";

Current numeric locale: C

<?php
$previous = setlocale(LC_NUMERIC, 0);
echo "Current numeric locale: $previous\n";

Current numeric locale: C

#[Attribute] on an abstract class, interface, trait or enum

php.net: “Applying #[\Attribute] to an abstract class, enum, interface, or trait triggers an error during compilation. Previously, the attribute could be added, but when ReflectionAttribute::newInstance() was called an error would be thrown.” (php.net)

In plain words: code that was only wrong in a corner case now stops the whole file from loading, even if that corner case never happens on your site.

<?php
#[Attribute]
abstract class Route {
}

echo "Routes loaded\n";

Routes loaded

Fatal error: Cannot apply #[\Attribute] to abstract class Route in /var/www/html/wp-content/plugins/shop-api/src/Route.php on line 3
Stack trace:
#0 {main}

<?php
abstract class Route {
}

#[Attribute]
final class Get extends Route {
}

echo "Routes loaded\n";

Routes loaded

class_alias() to array or callable

php.net: “It is no longer possible to use “array” and “callable” as class alias names in class_alias().” (php.net) PHP ignores upper and lower case here, so Array fails too.

<?php
class ArrayHelper {
}

class_alias('ArrayHelper', 'Array');
echo "Helpers loaded\n";

Helpers loaded

Fatal error: Cannot use "array" as a class alias as it is reserved in /var/www/html/wp-content/plugins/shop-api/helpers.php on line 5
Stack trace:
#0 /var/www/html/wp-content/plugins/shop-api/helpers.php(5): class_alias('ArrayHelper', 'Array')
#1 {main}

<?php
class ArrayHelper {
}

class_alias('ArrayHelper', 'Arr');
echo "Helpers loaded\n";

Helpers loaded

This is how a check reports the first case in a plugin:

What only shows notices: ten deprecations

A deprecation notice means “this still works, but a future PHP version is expected to change it.” The page is built as usual. PHP writes the notice to the error log, or onto the page if displaying errors is switched on, which should never be the case on a live site (WordPress debugging documentation).

PHP 8.5 deprecates more than ten things. These are the ten you’re most likely to meet in plugins and themes; php.net lists the full set in its deprecated features in PHP 8.5.

Notices can add up: in the readdir() example below, PHP writes the same notice once for every file in the folder.

The backtick operator

`command` runs a server command, as a shortcut for shell_exec(). Replace it with shell_exec().

<?php
$greeting = `echo hello`;
echo trim($greeting), "\n";

hello

Deprecated: The backtick (`) operator is deprecated, use shell_exec() instead in /var/www/html/example.php on line 2
hello

<?php
$greeting = shell_exec('echo hello');
echo trim($greeting), "\n";

hello

case followed by a semicolon

PHP has always accepted a semicolon after case (case 'x'; instead of case 'x':); PHP 8.5 deprecates it. Replace the semicolon with a colon; the same goes for default;.

<?php
$mode = 'compact';
switch ($mode) {
    case 'compact';
        echo "Compact layout\n";
        break;
    default;
        echo "Default layout\n";
}

Compact layout

Deprecated: Case statements followed by a semicolon (;) are deprecated, use a colon (:) instead in /var/www/html/example.php on line 4

Deprecated: Case statements followed by a semicolon (;) are deprecated, use a colon (:) instead in /var/www/html/example.php on line 7
Compact layout

<?php
$mode = 'compact';
switch ($mode) {
    case 'compact':
        echo "Compact layout\n";
        break;
    default:
        echo "Default layout\n";
}

Compact layout

Old cast names

(boolean), (integer), (double) and (binary) become (bool), (int), (float) and (string).

<?php
$value = '42 items';
$count = (integer) $value;
echo "Count: $count\n";

Count: 42

Deprecated: Non-canonical cast (integer) is deprecated, use the (int) cast instead in /var/www/html/example.php on line 3
Count: 42

<?php
$value = '42 items';
$count = (int) $value;
echo "Count: $count\n";

Count: 42

__debugInfo() returning null

This method tells PHP what to show when an object is inspected for debugging. Return an empty array instead of null.

<?php
class Cart {
    public function __debugInfo() {
        return null;
    }
}

var_dump(new Cart());

object(Cart)#1 (0) {
}

Deprecated: Returning null from Cart::__debugInfo() is deprecated, return an empty array instead in /var/www/html/example.php on line 8
object(Cart)#1 (0) {
}

<?php
class Cart {
    public function __debugInfo() {
        return [];
    }
}

var_dump(new Cart());

object(Cart)#1 (0) {
}

null as an array key

PHP has always stored null keys as an empty string; now it says so. Write ''.

<?php
$labels = [null => 'None', 'a' => 'Option A'];
echo $labels[''], "\n";

None

Deprecated: Using null as an array offset is deprecated, use an empty string instead in /var/www/html/example.php on line 2
None

<?php
$labels = ['' => 'None', 'a' => 'Option A'];
echo $labels[''], "\n";

None

setAccessible() on Reflection objects

It has done nothing since PHP 8.1. Call it only on older versions, or remove it.

<?php
class Order {
    private $total = 99;
}

$property = new ReflectionProperty(Order::class, 'total');
$property->setAccessible(true);
echo $property->getValue(new Order()), "\n";

99

Deprecated: Method ReflectionProperty::setAccessible() is deprecated since 8.5, as it has no effect since PHP 8.1 in /var/www/html/example.php on line 7
99

<?php
class Order {
    private $total = 99;
}

$property = new ReflectionProperty(Order::class, 'total');
if (PHP_VERSION_ID < 80100) {
    $property->setAccessible(true);
}
echo $property->getValue(new Order()), "\n";

99

$http_response_header

After fetching a web address, PHP used to fill this variable with the response headers. Use http_get_last_response_headers() (PHP 8.4 and later). The notice appears as soon as the file loads, even if the code never runs.

<?php
function fetch_status(string $url): string {
    file_get_contents($url);
    return $http_response_header[0];
}

echo "Loaded\n";

Loaded

Deprecated: The predefined locally scoped $http_response_header variable is deprecated, call http_get_last_response_headers() instead in /var/www/html/example.php on line 4
Loaded

<?php
function fetch_status(string $url): string {
    file_get_contents($url);
    $headers = function_exists('http_get_last_response_headers')
        ? http_get_last_response_headers()
        : ($http_response_header ?? []);
    return $headers[0] ?? '';
}

echo "Loaded\n";

Loaded

Clean-up functions that do nothing: curl_close() and others

curl_close(), curl_share_close(), imagedestroy() and xml_parser_free() have had no effect since PHP 8.0, and finfo_close() isn’t needed because PHP frees these objects automatically. Remove the call, or keep it only for older PHP versions. The aliases mysqli_execute() and socket_set_timeout() are deprecated too.

<?php
$handle = curl_init();
curl_close($handle);
echo "Done\n";

Done

Deprecated: Function curl_close() is deprecated since 8.5, as it has no effect since PHP 8.0 in /var/www/html/example.php on line 3
Done

<?php
$handle = curl_init();
if (PHP_VERSION_ID < 80000) {
    curl_close($handle);
}
echo "Done\n";

Done

DATE_RFC7231

php.net deprecates this constant “because the associated timezone is ignored and always uses GMT.” Write the format out, in GMT.

<?php
echo date(DATE_RFC7231, 0), "\n";

Thu, 01 Jan 1970 00:00:00 GMT

Deprecated: Constant DATE_RFC7231 is deprecated since 8.5, as this format ignores the associated timezone and always uses GMT in /var/www/html/example.php on line 2
Thu, 01 Jan 1970 00:00:00 GMT

<?php
echo gmdate('D, d M Y H:i:s \G\M\T', 0), "\n";

Thu, 01 Jan 1970 00:00:00 GMT

readdir() and friends without an argument

Without an argument, these functions used the last opened directory. Pass the directory handle explicitly.

<?php
$dir = opendir(__DIR__);
$count = 0;
while (false !== ($entry = readdir())) {
    $count++;
}
closedir();
echo "Entries: $count\n";

Entries: 3

Deprecated: readdir(): Passing null is deprecated, instead the last opened directory stream should be provided in /var/www/html/example.php on line 4

Deprecated: readdir(): Passing null is deprecated, instead the last opened directory stream should be provided in /var/www/html/example.php on line 4

Deprecated: readdir(): Passing null is deprecated, instead the last opened directory stream should be provided in /var/www/html/example.php on line 4

Deprecated: readdir(): Passing null is deprecated, instead the last opened directory stream should be provided in /var/www/html/example.php on line 4

Deprecated: closedir(): Passing null is deprecated, instead the last opened directory stream should be provided in /var/www/html/example.php on line 7
Entries: 3

<?php
$dir = opendir(__DIR__);
$count = 0;
while (false !== ($entry = readdir($dir))) {
    $count++;
}
closedir($dir);
echo "Entries: $count\n";

Entries: 3

What no code reader can see

Some PHP 8.5 changes depend on the values a plugin works with while it runs, so no tool that reads code can predict them. From php.net’s lists of incompatible changes and deprecations:

• “Incrementing non-numeric strings is now deprecated.”
• “Passing integers outside the interval [0, 255] to chr() is now deprecated.”
• null as an array key when the key comes from a variable.
• “Destructuring non-array values (other than null) using [] or list() now emits a warning.”
• “A warning is now emitted when casting floats (or strings that look like floats) to int if they cannot be represented as one.”

These produce deprecation notices or warnings, not fatal errors, but they are a good reason to look at your error log for a few days after switching.

How to check your site before switching

If something does break after the switch, “There has been a critical error” after a PHP update shows how to get your site back and find the cause. If your site is still on PHP 8.2, note its end-of-life date: PHP 8.2 reaches end of life on 31 December 2026.