PHP 8.4 compatibility checker for WordPress: what it must find, and how to run one

Why check for PHP 8.4

PHP 8.4 gets security fixes until 31 December 2028, a year longer than 8.3 (php.net). For a site leaving PHP 8.2, whose fixes end on 31 December 2026, it is a natural target. WordPress 7.0 and 7.1 support it (PHP requirements of WordPress 7); whether your site does depends on its plugins and theme.

What a PHP 8.4 checker must find

These are CompatNav’s 9 checks for PHP 8.4, each one a change from php.net’s lists of deprecated features and incompatible changes:

A good checker also skips code that already protects itself, for example with a PHP version check or function_exists(), so it doesn’t raise false alarms.

The one that stops code

A plugin that names one of the removed MySQLi constants runs on PHP 8.3 and stops on 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

A notice from everyday code

PHP 8.4 deprecates making a parameter nullable only through its default value, a way of writing that was normal for years. The code keeps working, with a notice each time the file loads:

<?php
class Order_Note {
    public function __construct(string $text, DateTime $created = null) {
        echo $text, $created ? ' (dated)' : ' (no date)', "\n";
    }
}

new Order_Note('Gift wrap');

Gift wrap (no date)

Deprecated: Order_Note::__construct(): Implicitly marking parameter $created as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/wp-content/plugins/order-notes/order-notes.php on line 3
Gift wrap (no date)

More on this one, with the fix: “Implicitly marking parameter as nullable is deprecated”. All the PHP 8.4 changes in plain words: WordPress and PHP 8.4.

What no checker can see

• Extensions. PHP 8.4 no longer includes IMAP, OCI8, PDO_OCI and PSpell. Whether your host still installs them for 8.4 is a server question, not a code one: IMAP and PHP 8.4 shows what happens when one is missing, and what to ask.
• Values. Some problems depend on the data a site handles while it runs. Your error log shows them after the switch (the WordPress debug log).

Run the check

CompatNav compares the PHP version your site runs today with the one you choose and reports only what changes between the two. From PHP 8.3, that’s the 9 checks above. From 8.2, the 4 checks for 8.3 as well, 13 in all. From 7.4, every check for 8.0 to 8.4: 45. Whatever the version, it also reports a file that PHP 8.4 can no longer read at all.

Nothing leaves your server: no code, file names or results are sent anywhere, and the check only reads files.