WordPress and PHP 8.4: what changes for plugins and themes

Is WordPress ready for PHP 8.4?

PHP 8.4.0 was released on 21 November 2024. It gets bug fixes until 31 December 2026 and security fixes until 31 December 2028 (php.net).

The WordPress core team’s compatibility page lists WordPress 6.7 and later as compatible with PHP 8.4. That covers WordPress itself; the PHP 8.4 changes below concern the code of your plugins and theme.

The short version

The PHP 8.4 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 can stop the code

A fatal error stops PHP at that point: the page isn’t built, and WordPress shows its critical-error message. Of the PHP 8.4 changes you’re most likely to meet in plugins and themes, two can cause one.

Extensions that are no longer part of PHP

IMAP, OCI8, PDO_OCI and PSpell “have been moved to PECL and are no longer part of the PHP distribution” (php.net). A plugin that uses PHP’s IMAP functions to read email, for example, works on PHP 8.4 only if your host installs that extension separately. That is a server setting, not something a check of your code can see, so ask your host before you switch.

Five removed MySQLi constants

PHP 8.4 removed five MySQLi constants that were never usable or only an alias, for example: “The unused MYSQLI_TYPE_INTERVAL constant, which is currently a stub and an alias for MYSQLI_TYPE_ENUM, has been removed.” (php.net) Code that still names one of them stops on PHP 8.4:

<?php
$field_type = 247; // the type number MySQL reports for a column
if ($field_type === MYSQLI_TYPE_INTERVAL) {
    echo "List column\n";
}
echo "Done\n";

List column
Done

Fatal error: Uncaught Error: Undefined constant "MYSQLI_TYPE_INTERVAL" in /var/www/html/wp-content/plugins/table-viewer/includes/columns.php:3
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/plugins/table-viewer/includes/columns.php on line 3

<?php
$field_type = 247; // the type number MySQL reports for a column
if ($field_type === MYSQLI_TYPE_ENUM) {
    echo "List column\n";
}
echo "Done\n";

List column
Done

This is how a check reports it in a plugin:

What only shows notices: the deprecations you’re most likely to meet

A deprecation notice means: this still works, but a future PHP version is expected to change it. The page is built as usual, and the notice goes to the error log, or onto the page if displaying errors is switched on, which should never be the case on a live site (how to keep them off your pages).

PHP 8.4 deprecates more than these eight; php.net lists the full set in its deprecated features in PHP 8.4.

Parameters with a null default but no ?

The notice reads “Implicitly marking parameter … as nullable is deprecated”. php.net explains: “A parameter’s type is implicitly widened to accept null if the default value for it is null.” (php.net) The fix is to say it in the type with ?, which works since PHP 7.1. The notice appears as soon as the file is compiled, even if the function is never called.

<?php
function format_price(float $amount, string $currency = null): string {
    return number_format($amount, 2) . ' ' . ($currency ?? 'EUR');
}

echo format_price(19.9), "\n";

19.90 EUR

Deprecated: format_price(): Implicitly marking parameter $currency as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/wp-content/plugins/price-format/includes/format.php on line 2
19.90 EUR

<?php
function format_price(float $amount, ?string $currency = null): string {
    return number_format($amount, 2) . ' ' . ($currency ?? 'EUR');
}

echo format_price(19.9), "\n";

19.90 EUR

trigger_error() with E_USER_ERROR

“Calling trigger_error() with error_level being equal to E_USER_ERROR is now deprecated. Such usages should be replaced by either throwing an exception, or calling exit(), whichever is more appropriate.” (php.net)

<?php
function check_licence($key) {
    if ($key === '') {
        trigger_error('A licence key is required.', E_USER_ERROR);
    }
    return true;
}

check_licence('');

Fatal error: A licence key is required. in /var/www/html/wp-content/plugins/licence-check/licence-check.php on line 4

Deprecated: Passing E_USER_ERROR to trigger_error() is deprecated since 8.4, throw an exception or call exit with a string message instead in /var/www/html/wp-content/plugins/licence-check/licence-check.php on line 4

Fatal error: A licence key is required. in /var/www/html/wp-content/plugins/licence-check/licence-check.php on line 4

<?php
function check_licence($key) {
    if ($key === '') {
        throw new InvalidArgumentException('A licence key is required.');
    }
    return true;
}

try {
    check_licence('');
} catch (InvalidArgumentException $e) {
    echo 'Licence problem: ', $e->getMessage(), "\n";
}

Licence problem: A licence key is required.

CSV functions without the escape argument

“Using the default value for the escape parameter for the fputcsv(), fgetcsv(), and str_getcsv() is now deprecated. It must be passed explicitly either positionally or via named arguments.” (php.net) Passing today’s default, a backslash, keeps the output exactly as it was.

<?php
$file = fopen('php://memory', 'r+');
fputcsv($file, ['Order', 'Total']);
rewind($file);
echo stream_get_contents($file);

Order,Total

Deprecated: fputcsv(): the $escape parameter must be provided as its default value will change in /var/www/html/wp-content/plugins/order-export/order-export.php on line 3
Order,Total

<?php
$file = fopen('php://memory', 'r+');
fputcsv($file, ['Order', 'Total'], ',', '"', '\\');
rewind($file);
echo stream_get_contents($file);

Order,Total

The E_STRICT constant

“Because the E_STRICT error level was removed, this constant is now deprecated.” (php.net) Old code often leaves it out of the error level; there is nothing left to leave out.

<?php
error_reporting(E_ALL & ~E_STRICT);
echo "Error level set\n";

Error level set

Deprecated: Constant E_STRICT is deprecated in /var/www/html/wp-content/plugins/debug-helper/debug-helper.php on line 2
Error level set

<?php
error_reporting(E_ALL);
echo "Error level set\n";

Error level set

lcg_value() and some mysqli_* functions

“lcg_value() is now deprecated, as the function is broken in multiple ways.” (php.net) The same page deprecates mysqli_ping(), mysqli_kill() and mysqli_refresh(), and xml_set_object().

<?php
$chance = lcg_value();
echo $chance < 1 ? "Random number OK\n" : "";

Random number OK

Deprecated: Function lcg_value() is deprecated since 8.4, use \Random\Randomizer::getFloat() instead in /var/www/html/wp-content/plugins/random-quote/random-quote.php on line 2
Random number OK

<?php
$chance = mt_rand() / mt_getrandmax();
echo $chance <= 1 ? "Random number OK\n" : "";

Random number OK

session_set_save_handler() with separate functions

“Calling session_set_save_handler() with more than two arguments is deprecated.” (php.net) The two-argument form takes an object that implements SessionHandlerInterface.

<?php
session_set_save_handler(
    function ($path, $name) { return true; },
    function () { return true; },
    function ($id) { return ''; },
    function ($id, $data) { return true; },
    function ($id) { return true; },
    function ($max_lifetime) { return 0; }
);
echo "Session storage set\n";

Session storage set

Deprecated: session_set_save_handler(): Providing individual callbacks instead of an object implementing SessionHandlerInterface is deprecated in /var/www/html/wp-content/plugins/member-area/includes/sessions.php on line 2

Warning: session_set_save_handler(): Session save handler cannot be changed after headers have already been sent in /var/www/html/wp-content/plugins/member-area/includes/sessions.php on line 2
Session storage set

<?php
class Member_Session_Handler implements SessionHandlerInterface {
    public function open(string $path, string $name): bool { return true; }
    public function close(): bool { return true; }
    public function read(string $id): string|false { return ''; }
    public function write(string $id, string $data): bool { return true; }
    public function destroy(string $id): bool { return true; }
    public function gc(int $max_lifetime): int|false { return 0; }
}

session_set_save_handler(new Member_Session_Handler(), true);
echo "Session storage set\n";

Session storage set

A class named _

“Naming a class _ is now deprecated” (php.net); names that only start with an underscore are fine.

<?php
class _ {
    public static function text($text) {
        return $text;
    }
}

echo _::text('Read more'), "\n";

Read more

Deprecated: Using "_" as a class name is deprecated since 8.4 in /var/www/html/wp-content/themes/tiny-theme/inc/translate.php on line 2
Read more

<?php
class Tiny_Theme_Text {
    public static function text($text) {
        return $text;
    }
}

echo Tiny_Theme_Text::text('Read more'), "\n";

Read more

Changed return types of built-in methods

A plugin class that extends a PHP class and overrides one of its methods must promise a compatible return type. PHP 8.4 narrowed some of these types, for example DateTime::modify(): “Now has a tentative return type of DateTime. Previously it was DateTime|false.” (php.net) An override written for PHP 8.3 then gets a notice.

<?php
class Event_Date extends DateTime {
    public function modify(string $modifier): DateTime|false {
        return parent::modify($modifier);
    }
}

$date = new Event_Date('2026-05-01');
echo $date->modify('+1 day')->format('j F Y'), "\n";

2 May 2026

Deprecated: Return type of Event_Date::modify(string $modifier): DateTime|false should either be compatible with DateTime::modify(string $modifier): DateTime, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /var/www/html/wp-content/plugins/event-calendar/includes/class-event-date.php on line 3
2 May 2026

<?php
class Event_Date extends DateTime {
    public function modify(string $modifier): DateTime {
        return parent::modify($modifier);
    }
}

$date = new Event_Date('2026-05-01');
echo $date->modify('+1 day')->format('j F Y'), "\n";

2 May 2026

2 May 2026

2 May 2026

2 May 2026

What no code reader can see

Some PHP 8.4 changes depend on the values a plugin works with while it runs. For example, raising zero to a negative power “corresponds to dividing by 0 … Thus, this behavior has been deprecated.”, and “Passing invalid options to hash functions is now deprecated.” (php.net) Whether that happens depends on the data, so look at your error log for a few days after switching.

How to check your site before switching

PHP 8.5 changes more; if your host offers it, compare with WordPress and PHP 8.5: what breaks, what only shows notices. Which version to choose and until when each gets security fixes: PHP 8.2 reaches end of life on 31 December 2026.