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PHP Fatal Error: Cannot Declare Class WP_Metadata_Lazyloader — Fix Guide

Seeing “PHP Fatal error: Cannot declare class WP_Metadata_Lazyloader, because the name is already in use” after upgrading WordPress to 6.3? This guide explains what causes the error and walks through the fix step by step. It typically appears during a WordPress version migration from 6.2.2 to 6.3, in your site’s error log or as a white screen on both the frontend and wp-admin.

The full error usually looks like this:

“PHP Fatal error: Cannot declare class WP_Metadata_Lazyloader, because the name is already in use in /home/xxxx/public_html/wp-includes/class-wp-metadata-lazyloader.php on line 32”

What Causes This Error?

The error means WordPress is loading the WP_Metadata_Lazyloader class definition twice. PHP does not allow the same class to be declared two times in one request, so the process terminates with a fatal error.

In practice this happens when the WordPress 6.3 upgrade is incomplete or partial — core files from 6.2.2 and 6.3 end up mixed in the same installation. Specifically:

  • The class file class-wp-metadata-lazyloader.php is required more than once, or
  • Autoload and manual-require paths in wp-settings.php both load the same class file.

The WordPress core team tracked this issue in ticket #59057 on WordPress Trac.

How to Fix It: Step by Step

Step 1 — Check wp-settings.php Line 191

Open wp-settings.php in your WordPress root folder and look at around line 191. If you find a manual require of “class-wp-metadata-lazyloader.php”** there, it means the 6.3 upgrade was applied incorrectly — that line should not exist in a clean 6.3 install.

WordPress wp-settings.php fatal error — duplicate class-wp-metadata-lazyloader.php require on line 191

Step 2 — Back Up Your Files

Before changing anything, create a full backup. Use FTP/SFTP or your hosting file manager and copy the site root (at minimum: wp-settings.php, wp-includes/, wp-admin/).

Step 3 — Replace wp-settings.php With a Clean Copy

Download fresh WordPress core files from wordpress.org/download/, then manually replace the wp-settings.php file in your root folder with the one from the download. This removes the duplicated require and usually restores access to both the site and wp-admin immediately.

Step 4 — Reinstall WordPress 6.3 Properly

  1. Grant write permissions to the wp-admin folder (for example 755).
  2. Log in to the admin panel and run the update to 6.3 again (Dashboard → Updates → Re-install version 6.3).
  3. When the reinstall finishes, verify the site and admin both work.
  4. Revert the wp-admin folder permissions to read-only.

Step 5 — If the Entry Is Missing (Opposite Scenario)

If step 1 found no “class-wp-metadata-lazyloader.php” entry in wp-settings.php but the error still occurs, apply the official patch from the WordPress core team: wordpress-develop PR #4992.

How to Prevent This in Future Upgrades

  • Never interrupt a WordPress update — a stopped upgrade is the most common cause of mixed core files.
  • Take a backup before upgrading (files + database), so rollback is quick.
  • Prefer one-click admin updates or WP-CLI (wp core update) over manually uploading individual core files.
  • After any update, check Tools → Site Health for verification that core files match the release.
  • Staging first: test major version upgrades on a staging copy before applying to production.

Still Stuck?

If the error persists after these steps, the installation may have deeper file corruption. Contact Cognic — our team fixes WordPress migration failures, broken upgrades, and hosting-level PHP issues regularly, and can restore your site quickly.

Frequently Asked Questions

Common questions about the WP_Metadata_Lazyloader fatal error:

What does “Cannot declare class WP_Metadata_Lazyloader” mean?
It means PHP tried to define the WP_Metadata_Lazyloader class twice in the same request. PHP forbids duplicate class declarations, so execution halts with a fatal error. On WordPress sites it almost always indicates mixed core files after a partial or interrupted upgrade to 6.3.
Is this error caused by a plugin or theme?
Usually no. The error references a WordPress core file (wp-includes/class-wp-metadata-lazyloader.php). It stems from the WordPress upgrade process itself, though a badly coded plugin that loads core files early could theoretically trigger it — check wp-settings.php first as described in the guide.
Will replacing wp-settings.php break my site?
No, as long as you replace it with the wp-settings.php from the exact WordPress version you are running (6.3). Always back up the old file first. The file contains only core bootstrap logic — it has no site-specific configuration.
Why did my WordPress 6.3 upgrade fail partially?
Common causes: the update was interrupted (browser closed, network drop, server timeout), file permissions blocked writing to some folders, disk space ran out mid-copy, or a caching/OPcache layer served stale files during the upgrade.
Can I just downgrade to WordPress 6.2.2 instead?
You can restore 6.2.2 from a backup, but it is a temporary escape. The cleaner fix is completing the 6.3 reinstall as described above, because a mixed installation will keep producing errors on future updates too.