Maintaining a WordPress website using a custom-built theme comes with its share of rewards—and responsibilities. It offers unique design flexibility and tailored functionality, but when a major WordPress core update rolls out, vulnerabilities and compatibility issues can suddenly rise to the surface. This is exactly what happened to me when my custom theme abruptly stopped working after a recent core update. What followed was a meticulous version compatibility audit to bring my site back online and prevent future breakdowns.
TL;DR (Too long, didn’t read)
When my custom WordPress theme failed after a core update, it disrupted my live site and triggered an immediate troubleshooting process. I performed a detailed version compatibility audit, identifying functions and template parts affected by the changes in the new WordPress core. The issue stemmed from deprecated functions, outdated practices, and some overlooked plugin conflicts. After applying code adjustments, testing in a staging environment, and updating legacy scripts, I restored full compatibility while documenting vital takeaways for future-proof maintenance.
The Shock: My Theme Suddenly Fails Post Update
The day started like any other, only slightly more productive—I had just completed the update to the latest version of WordPress. The changelog looked exciting, boasting performance enhancements and better PHP 8.1 support. But a few minutes post-update, I noticed that several pages were returning a “white screen of death.” The login page was fine, but the homepage and some custom post-type templates were either malfunctioning or completely broken.
For anyone using a commercial theme or a widely-supported framework, such issues might be rare or quickly resolved by the developers. But for my custom-built theme, built three years earlier, the documentation was sparse, and the original developer had long moved on.
It was time for a deep dive into compatibility checks.
Initial Troubleshooting Steps
Here’s a quick rundown of the steps I initially took:
- Enabled WP_DEBUG mode in
wp-config.phpto see error messages - Checked Logs: Noticed numerous warnings and a few fatal errors pointing to deprecated functions
- Switched to Default Theme: Confirmed that WordPress core was working fine
- Disabled All Plugins: Isolated that the issue was mostly within the theme itself
The problem was clearly within the theme’s codebase—more specifically, outdated functions that no longer existed or were modified in the updated core.
Conducting a Version Compatibility Audit
I created a structured plan to audit the theme’s compatibility with the updated WordPress core. The audit involved reviewing functions, template hierarchies, JavaScript dependencies, and third-party integrations.
1. Review Deprecated Functions
The “call to undefined function” errors were my first clue. I compared the theme’s functions.php and template files against the updated list of WordPress deprecated functions found in the official WordPress developer handbook.
Notable changes included:
- Use of
get_theme_data()(deprecated as of WordPress 3.4) - Custom walker classes relying on outdated
Walker_Pagestructures - Old action hooks that had either been renamed or removed
2. Compatibility Layer Check
The next step was searching the theme for functions that heavily interacted with the core WordPress APIs. These included:
- REST API endpoints
- Customizer APIs
- Editor (Gutenberg) integrations
WordPress now expects tighter TypeScript compliance within custom JavaScript files, and my theme scripts were still using legacy jQuery syntax and no module separation.
3. PHP Compatibility Review
Another major issue was related to PHP version mismatches. The theme had several anonymous functions and closures that caused syntax issues under PHP 8.1, especially where type declarations were improperly used. Using tools like PHPCompatibilityWP, I audited over 3000 lines of code and replaced legacy calls with updated patterns.
4. Template Hierarchy Follow-up
I examined template files like single.php, page.php, and custom templates for naming consistency and fallback behavior. A missed update in the get_template_part() pattern caused major layout issues. I replaced hardcoded paths with dynamic calls to improve resilience.
Patch, Test, Repeat
After cleaning up deprecated functions, correcting PHP syntax violations, and updating JavaScript libraries, I created a staging environment to validate changes. This isolated setup mirrored production while allowing me to test the updated theme layer by layer.
Steps taken in staging:
- Migrated full site content with WP-CLI
- Used a diff-checker to compare function outputs line-by-line
- Ran Lighthouse audits to verify performance wasn’t compromised
Most important: I maintained a changelog to clearly document which files had been touched and why—a crucial step for any future audits or developer handoffs.
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Plugin Conflicts: A Hidden Culprit
During testing, I discovered the site’s SEO plugin was throwing fatal errors due to conflicts with the modified theme header. The theme used custom implementations of wp_head and wp_footer which were not calling essential plugin actions.
By adding:
<?php wp_head(); ?>and
<?php wp_footer(); ?>back into the respective template files, I restored much of the plugin functionality.
Sometimes, fixing a theme is not just about the theme itself but also ensuring it respects WordPress’s global hooks and plugin architecture.
Key Lessons and Takeaways
This compatibility audit wasn’t just a reactive fix; it was a wake-up call to proactively manage theme development against an evolving platform. Here are several lessons that stood out:
- Custom code needs lifecycle monitoring—not just building but maintaining it over time.
- Always audit code after major updates, even if the site “seems” to be working.
- Use staging environments to test issues without affecting live traffic.
- Stay current with core WordPress changes by participating in release candidate (RC) testing when you rely on custom builds.
The Final Fix
Over a span of four days, I repaired all deprecated dependencies, reinforced best practices, and brought my custom theme up to compatibility with WordPress 6.4. I also used this opportunity to migrate some hardcoded features—like social embeds and email obfuscation—to shortcodes and dedicated plugins to reduce theme complexity.
Now, my site is fully functional, faster than before, and prepared for the next major update. It’s never stress-free when something breaks, but with the right process in place, recovery is possible.
Conclusion
If you’re relying on a custom theme in WordPress, make version compatibility audits part of your development hygiene. Changes in the core often impact legacy code in unexpected ways. By staying informed, maintaining clean code, and setting up a proper testing pipeline, you’ll mitigate the risk of site-wide outages and ensure your website continues to perform at its best.
