WordPress and PHP 8.3: what changes for plugins and themes

Is WordPress ready for PHP 8.3?

PHP 8.3.0 was released on 23 November 2023. Its active support ended on 31 December 2025; it gets security fixes until 31 December 2027 (php.net).

The WordPress core team’s compatibility page lists WordPress 6.4 and later as compatible with PHP 8.3. That covers WordPress itself; the changes below concern the code of your plugins and theme.

The short version

The PHP 8.3 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: only with some values

Of the PHP 8.3 changes you’re most likely to meet in plugins and themes, none stops code in every case. One can, depending on the values it gets. php.net’s changelog for DateTime::modify(): “DateTime::modify() now throws DateMalformedStringException if an invalid string is passed. Previously, it returned false, and a warning was emitted.” (php.net)

So a date a visitor typed, or a setting left in an odd format, used to give a warning and let the plugin carry on; on PHP 8.3 it stops the page unless the plugin catches the exception:

<?php
$reminder = new DateTime('2026-05-01');
$result = $reminder->modify('next thursday-ish'); // a date a visitor typed

echo $result ? $result->format('j F Y') : 'Not a valid date', "\n";

Warning: DateTime::modify(): Failed to parse time string (next thursday-ish) at position 13 (-): Unexpected character in /var/www/html/wp-content/plugins/event-calendar/includes/reminders.php on line 3
Not a valid date

Fatal error: Uncaught DateMalformedStringException: DateTime::modify(): Failed to parse time string (next thursday-ish) at position 13 (-): Unexpected character in /var/www/html/wp-content/plugins/event-calendar/includes/reminders.php:3
Stack trace:
#0 /var/www/html/wp-content/plugins/event-calendar/includes/reminders.php(3): DateTime->modify('next thursday-i...')
#1 {main}
  thrown in /var/www/html/wp-content/plugins/event-calendar/includes/reminders.php on line 3

<?php
$reminder = new DateTime('2026-05-01');
try {
    $result = $reminder->modify('next thursday-ish'); // a date a visitor typed
} catch (Exception $e) { // PHP 8.3 and later throw here
    $result = false;
}

echo $result ? $result->format('j F Y') : 'Not a valid date', "\n";

Warning: DateTime::modify(): Failed to parse time string (next thursday-ish) at position 13 (-): Unexpected character in /var/www/html/wp-content/plugins/event-calendar/includes/reminders.php on line 4
Not a valid date

Not a valid date

No code check can tell in advance which dates a site will pass, so watch your error log after switching.

A silent change: negative array indexes

“Assigning a negative index $n to an empty array will now make sure that the next index is $n+1 instead of 0.” (php.net) No notice, no error; only the keys change:

<?php
$ranking = [];
$ranking[-3] = 'Guest';
$ranking[] = 'Adam';
$ranking[] = 'Sara';

echo implode(', ', array_keys($ranking)), "\n";

-3, 0, 1

-3, -2, -1

Code that relies on the old numbering (for example, looking for the first item at index 0) gets a different result without any message.

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. PHP 8.3 deprecates more than these; php.net lists the full set in its deprecated features in PHP 8.3.

get_class() without arguments

“Calling get_class() and get_parent_class() without arguments is now deprecated.” (php.net) The fix, with tested code, is in PHP 8.2 reaches end of life: self::class inside the class.

assert_options() and the ASSERT_* constants

“The assert_options() function is now deprecated. The ASSERT_ACTIVE, ASSERT_BAIL, ASSERT_CALLBACK, ASSERT_EXCEPTION, and ASSERT_WARNING constants have been deprecated.” (php.net) A line using both gets two notices; PHP 8.5 words them differently:

<?php
if (assert_options(ASSERT_ACTIVE)) {
    echo "Assertions are on\n";
}

Assertions are on

Deprecated: Constant ASSERT_ACTIVE is deprecated in /var/www/html/wp-content/plugins/dev-checks/dev-checks.php on line 2

Deprecated: Function assert_options() is deprecated in /var/www/html/wp-content/plugins/dev-checks/dev-checks.php on line 2
Assertions are on

Deprecated: Constant ASSERT_ACTIVE is deprecated since 8.3, as assert_options() is deprecated in /var/www/html/wp-content/plugins/dev-checks/dev-checks.php on line 2

Deprecated: Function assert_options() is deprecated since 8.3 in /var/www/html/wp-content/plugins/dev-checks/dev-checks.php on line 2
Assertions are on

This is how a check reports it:

<?php
if (ini_get('assert.active')) {
    echo "Assertions are on\n";
}

Assertions are on

Assertions are on

MT_RAND_PHP

“The MT_RAND_PHP Mt19937 variant is deprecated.” (php.net) Worth knowing before fixing it: the default variant gives a different sequence of numbers for the same seed, so a plugin that relied on a fixed “random” order shows a different one.

<?php
mt_srand(42, MT_RAND_PHP); // the same "random" quote for everyone today
echo 'Quote number ', mt_rand(1, 100), "\n";

Quote number 64

Deprecated: Constant MT_RAND_PHP is deprecated in /var/www/html/wp-content/plugins/daily-quote/daily-quote.php on line 2

Deprecated: The MT_RAND_PHP variant of Mt19937 is deprecated in /var/www/html/wp-content/plugins/daily-quote/daily-quote.php on line 2
Quote number 64

Deprecated: Constant MT_RAND_PHP is deprecated since 8.3, as it uses a biased non-standard variant of Mt19937 in /var/www/html/wp-content/plugins/daily-quote/daily-quote.php on line 2

Deprecated: The MT_RAND_PHP variant of Mt19937 is deprecated in /var/www/html/wp-content/plugins/daily-quote/daily-quote.php on line 2
Quote number 64

<?php
mt_srand(42); // the same "random" quote for everyone today
echo 'Quote number ', mt_rand(1, 100), "\n";

Quote number 43

Quote number 43

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 8.3 narrowed some, for example ArrayObject::asort(), which now returns true; an override that still promises bool gets a notice:

<?php
class Sorted_List extends ArrayObject {
    public function asort(int $flags = SORT_REGULAR): bool {
        return parent::asort($flags);
    }
}

$list = new Sorted_List(['b' => 'Sara', 'a' => 'Adam']);
$list->asort();
echo implode(', ', (array) $list), "\n";

Adam, Sara

Deprecated: Return type of Sorted_List::asort(int $flags = SORT_REGULAR): bool should either be compatible with ArrayObject::asort(int $flags = SORT_REGULAR): true, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /var/www/html/wp-content/plugins/member-list/includes/class-sorted-list.php on line 3
Adam, Sara

<?php
class Sorted_List extends ArrayObject {
    public function asort(int $flags = SORT_REGULAR): true {
        return parent::asort($flags);
    }
}

$list = new Sorted_List(['b' => 'Sara', 'a' => 'Adam']);
$list->asort();
echo implode(', ', (array) $list), "\n";

Adam, Sara

Adam, Sara

Adam, Sara

++ on empty or non-alphanumeric strings

“Using the increment operator (++) on empty, non-numeric, or non-alphanumeric strings is now deprecated.” (php.net) Whether it happens depends on the string’s value at run time; PHP 8.5 words the message differently:

<?php
$code = ''; // no code saved yet
$code++;
var_dump($code);

string(1) "1"

Deprecated: Increment on non-alphanumeric string is deprecated in /var/www/html/wp-content/plugins/member-list/includes/codes.php on line 3
string(1) "1"

Deprecated: Increment on non-numeric string is deprecated, use str_increment() instead in /var/www/html/wp-content/plugins/member-list/includes/codes.php on line 3
string(1) "1"

How to check your site before switching

A newer version gives you more time before the next upgrade: WordPress and PHP 8.4 and WordPress and PHP 8.5 list what changes there.