What this problem is
After a PHP upgrade/downgrade (or on a new host), WordPress shows fatal errors, blank pages, or warnings that block the site.
Why it happens
- Your WordPress core, plugins, or theme do not support the active PHP version
- Deprecated functions removed in newer PHP versions
- Extensions required by a plugin are missing
Prerequisites
- Hosting panel access to change PHP version (MultiPHP, Select PHP Version, etc.)
- FTP/File Manager access (recommended)
Diagnosis
- Check the error message in logs. Common:
Fatal error,Call to undefined function,TypeError. - Confirm current PHP version in hosting panel.
Detailed steps
Step 1) Switch PHP version to a supported one
In your hosting panel, set PHP to a version compatible with your WordPress and plugins (commonly PHP 8.1/8.2 for modern sites, but depends on plugins).
Step 2) Disable the suspected plugin/theme if the site is still broken
Disable plugins by renaming wp-content/plugins, or switch theme by renaming the active theme folder.
Step 3) Update WordPress/plugins/themes
Once the site is stable, update components to versions compatible with your target PHP version.
Expected results
- Site loads without fatal errors
- You can move to a newer PHP version after updating incompatible components
What to do if it fails
- Ask the host for the exact PHP error log entry
- Install missing PHP extensions required by the plugin (host support)
- Replace the incompatible plugin/theme with an alternative
Best practices
- Test PHP upgrades on staging
- Keep plugins/themes updated to maintain PHP compatibility
- Remove abandoned plugins that do not track PHP versions