Using a PHP version incompatible with active plugins or themes.
PHP Version Mismatch
Key Points: Your site breaks because you’re using a PHP version that your theme or plugins aren’t ready for.
You upgraded PHP to the latest — great for performance! But now half your site is broken. Deprecated functions, white screens, and fatal errors greet you. Welcome to the chaos of PHP version mismatches.
🧨 Why It Happens
- Plugins still rely on old PHP functions removed in newer versions
- The theme hasn’t been updated in years
- You’re using beta PHP releases not supported by WordPress
🛠️ Solution
- Switch to a stable PHP version (usually 8.0–8.2 in 2025)
- Update all plugins and themes to their latest versions
- Check PHP compatibility using a plugin like PHP Compatibility Checker
- Temporarily activate default theme (like Twenty Twenty-Four) to isolate the issue
💡 Developer Tip
Never upgrade PHP on live sites without testing in staging first. Even one incompatible plugin can crash the whole site.