WordPress Broke After a PHP 8.4 Update

Before debugging, get the site up. Switch the PHP version back to the last one that worked in your host panel. On cPanel hosts, open MultiPHP Manager, select the domain, choose the previous version, and apply. On SiteGround, open Site Tools then Devs then PHP Manager and set a specific older version instead of managed. The change takes a minute or two.

This is triage, not a cure. The old version stops receiving security patches, so use the breathing room to fix the real cause and then move forward to PHP 8.3 or 8.4 deliberately, on staging first.

Update the named plugin or theme first. Most active plugins shipped PHP 8.4 compatible releases through 2025 and 2026, so an available update often resolves it outright.

If no update exists and the plugin has not been touched in over a year, treat it as abandoned and replace it with a maintained alternative. Holding an entire site on an end-of-life PHP version for one stale plugin is the wrong trade. If the fatal is in a custom theme or a paid plugin you cannot replace, the offending code needs a direct fix, for example making a parameter explicitly nullable.

// Triggers the PHP 8.4 deprecation, and on stricter setups a type error
function example( array $value = null ) { /* ... */ }

// PHP 8.4 safe, explicit nullable type
function example( ?array $value = null ) { /* ... */ }

The site is technically running, but notices are leaking into responses. First, make sure production is not displaying errors. In wp-config.php:

define( 'WP_DEBUG', false );
@ini_set( 'display_errors', 0 );

To keep real warnings visible while silencing only the deprecation flood from outdated plugins, drop a must-use plugin in wp-content/mu-plugins/quiet-deprecations.php that removes only the deprecation level:

<?php
// Suppress only E_DEPRECATED, keep warnings and errors visible.
// Temporary measure while waiting on plugin and theme updates.
error_reporting( error_reporting() & ~E_DEPRECATED );

This quiets the noise without hiding genuine problems. It is a stopgap. The real resolution is updating the plugins and themes that emit the notices.

The new PHP build is missing an image extension. Ask your host to enable gd or imagick on the active PHP version. Most hosts expose extension toggles in the same panel as the version selector, or will flip it for you on request. Confirm the result in Tools then Site Health then Info then Media, which should list an available image editor. If a plugin needs a different extension such as intl or zip, enable that the same way.

1. Clone the site to staging, or spin up a staging copy from your host.
2. Switch only staging to PHP 8.4 and exercise every important flow: login, checkout, forms, the editor, media uploads.
3. Watch wp-content/debug.log for fatals, and resolve each by updating or replacing the plugin or theme named in the path.
4. Once staging is clean, switch production to PHP 8.4 during a low-traffic window and watch the log for an hour.
5. Keep PHP current from then on. Sitting on the version your host forces you off is what creates the emergency in the first place.