How to Resolve WordPress Critical Error After PHP Upgrade: A Step-by-Step Guide

Direct Answer: How to Fix WordPress Critical Errors After a PHP Upgrade

A WordPress critical error immediately after upgrading PHP usually indicates a compatibility problem between the new PHP version and your WordPress core, theme, or plugins. The quickest way to resolve this is to temporarily roll back to your previous PHP version and then systematically troubleshoot the issue by enabling debug logs and disabling incompatible plugins or themes. Once identified, upgrade or replace faulty components before attempting the PHP upgrade again.

Prerequisites and Safety Checks Before Troubleshooting

• Backup Your Website: Before taking any action, create a full backup of your WordPress files and database. This ensures you can restore your site if anything goes wrong during troubleshooting.
• Access to Hosting Control Panel: Verify you have access to your hosting control panel or server management tools to change PHP versions, file manager or FTP, and error logs access.
• Knowledge of Your Current WordPress Environment: Know your current PHP version, WordPress version, and the list of active plugins and themes.
• Enable WordPress Debugging: Prepare to enable WordPress debugging to log detailed error information, which is crucial for pinpointing incompatibilities.

Step-by-Step Instructions to Resolve the Critical Error

1. Rollback to Previous PHP Version

   If the error started immediately after a PHP upgrade, revert to the previous known working PHP version using your hosting control panel. Most hosting providers have a PHP selector. If unsure, contact your host support.

2. Check If the Site Restores Normally

   After rollback, verify if the site functions correctly without the critical error. If yes, the issue is very likely a PHP compatibility problem.

3. Enable Debugging in WordPress

   Modify your wp-config.php file to enable debugging by adding or editing these lines before the /* That's all, stop editing! */ comment:

   define( 'WP_DEBUG', true );
   define( 'WP_DEBUG_LOG', true );
   define( 'WP_DEBUG_DISPLAY', false );

   This directs WordPress to log errors to wp-content/debug.log without displaying them on the frontend.

4. Switch to a Default Theme

   Temporarily switch your active theme to a default WordPress theme like “Twenty Twenty-One” or “Twenty Twenty-Three” via the admin dashboard or by renaming your current theme folder via FTP.

5. Deactivate All Plugins

   Deactivate all plugins by bulk action in the admin or by renaming the wp-content/plugins folder to something like plugins-deactivated. This helps identify if a plugin causes the error.

6. Incrementally Reactivate Plugins

   Rename the plugins-deactivated folder back to plugins and activate plugins one-by-one. After activating each plugin, check for the critical error by switching PHP to the newer version.

7. Upgrade or Replace Incompatible Plugins or Themes

   If a plugin or theme causes the error under the new PHP version, check if updates are available. If none, consider replacing it with an up-to-date alternative.

8. Update WordPress Core

   Ensure your WordPress core is updated to the latest version compatible with your desired PHP version.

9. Attempt the PHP Upgrade Again

   After resolving incompatibilities, upgrade PHP again and verify the site functions without the critical error.

How to Verify That the Issue Is Resolved

• Clear your browser and any caching plugins’ cache.
• Visit multiple pages including the WordPress admin area to check for errors.
• Review the wp-content/debug.log file for ongoing PHP notices or fatal errors.
• Confirm that all active plugins and themes are loading correctly.

Common Mistakes to Avoid

• Not Backing Up Prior to Changes: Running debugging or PHP changes without backups risks site loss.
• Ignoring Plugin or Theme Incompatibilities: Upgrading PHP without checking compatibility leads to critical errors.
• Not Reviewing Debug Logs: Skipping log checks can make troubleshooting impossible.
• Forgetting to Disable Debugging on production after resolving issues, which may expose sensitive info.
• Changing PHP Versions Without Using Host Tools: Manually editing server files without expertise can break the site further.

Alternatives If You Can’t Immediately Resolve the Critical Error

• Contact Hosting Support: Many hosts can assist with safe rollback and debugging.
• Restore a Previous Backup: If available, restore to a working backup made before the PHP upgrade.
• Use WP-CLI: If you have server access, use WP-CLI commands to deactivate plugins or switch themes offline.
• Hire a Developer: For complex issues involving custom code, professional help might be needed.

Example: Enabling Debugging and Checking Logs

// wp-config.php
// Add these lines:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

// After triggering the error,
// check the file at wp-content/debug.log for errors like:
// PHP Fatal error:  Uncaught Error: Call to undefined function ...

FAQ

Using WP-CLI to Troubleshoot WordPress Critical Errors

If you have SSH access to your server, the WP-CLI (WordPress Command Line Interface) tool can be invaluable for troubleshooting WordPress critical errors caused by PHP upgrades. WP-CLI lets you manage your WordPress installation without a web interface, allowing you to deactivate plugins or switch themes even when the site is broken.

1. Check WP-CLI Availability

   Log in to your server via SSH and run:

   wp --info

   If WP-CLI is installed, you will see version and environment info. If not, consider installing it following instructions at wp-cli.org or ask your host.

2. Deactivate All Plugins

   Execute the command:

   wp plugin deactivate --all

   This disables all plugins and lets you test PHP upgrades safely.

3. Switch to a Default Theme

   Run:

   wp theme activate twentytwentythree

   This activates the Twenty Twenty-Three theme, often compatible with latest PHP versions.

4. Upgrade PHP Version

   Once plugins are off and a default theme is active, attempt to upgrade PHP again via your hosting panel.

5. Test Site Accessibility

   Visit your site and admin dashboard to confirm if the critical error persists.

6. Reactivate Plugins One-by-One Offline

   To find the problematic plugin remotely, reactivate plugins one at a time via WP-CLI:

   wp plugin activate plugin-name

   Replace plugin-name with each plugin slug and check the site status between activations.

Troubleshooting Edge Cases and Complex Scenarios

Site Remains Down Despite Reverting PHP Version

If your site still shows a critical error even after downgrading PHP back to the previous version, consider these steps:

• Corrupted .htaccess File: Rename the .htaccess file in your WordPress root directory to .htaccess_backup to disable it temporarily and see if this resolves the error.
• Database Issues: Run a database repair by adding the following line to your wp-config.php file and then navigating to yourdomain.com/wp-admin/maint/repair.php:

  define('WP_ALLOW_REPAIR', true);

• File Permission Problems: Incorrect permissions can prevent WordPress from working properly. Verify that directories have 755 permissions, files have 644, and the server user owns them.
• Memory Exhaustion: Increase PHP memory limit by adding this to wp-config.php:

  define('WP_MEMORY_LIMIT', '256M');

White Screen of Death Instead of Critical Error Message

A blank white screen with no message often means a PHP fatal error without error display enabled. Enabling WP_DEBUG and checking logs as discussed earlier is essential. Also, check your server’s error logs via your hosting panel or via SSH:

• Look in /var/log/apache2/error.log or /var/log/nginx/error.log depending on your web server.
• Seek errors referencing plugins, themes, or syntax errors.

How to Safely Upgrade PHP for WordPress in the Future

1. Backup Everything: Always begin with full backups.
2. Set Up a Staging Environment: Create an exact copy of your website on a staging server to test PHP upgrades without affecting live users.
3. Verify Plugin and Theme Compatibility: Check each plugin’s and theme’s page for PHP version support. Use tools like PHP Compatibility Checker plugin on the staging site.
4. Upgrade Plugins, Themes, and WordPress Core: Make sure all are updated to their latest versions.
5. Perform PHP Upgrade on Staging Site First: Change PHP version and evaluate the site’s behavior in detail.
6. Fix Issues and Retest: Address incompatibilities or errors before planning upgrade on production.
7. Schedule Maintenance Window: Upgrade PHP on live site during low traffic periods to minimize impact.
8. Monitor the Site After Upgrade: Watch for errors and performance issues for several days post-upgrade.

Additional FAQ