WordPress Error - How to Fix 'TypeError: substr(): Argument #1 ($string) must be of type string, int given' in WP Rocket

If you updated your WordPress core to WordPress 7.1 and your site immediately crashed with the standard WordPress critical error screen:

There has been a critical error on this website. Learn more about troubleshooting WordPress.

And your PHP error_log displays a traceback pointing to WP Rocket's Cloudflare integration:

PHP Fatal error: Uncaught TypeError: substr(): Argument #1 ($string) must be of type string, int given in /public_html/wp-content/plugins/wp-rocket/inc/ThirdParty/Plugins/CDN/Cloudflare.php on line 496

Do not panic. Your database is completely safe, and your custom caching configurations are intact. This is a known, live compatibility bug between WordPress 7.1's updated hook architecture and WP Rocket's Cloudflare integration.

Here is the exact technical explanation of why this error happens and the step-by-step workflow to restore your site immediately.

The Technical Root Cause

To understand why this error crashes both your frontend and the /wp-admin/ login panel, we have to look at how WordPress 7.1 handles action and filter hooks.

1. Hook Callback ID Shift

WordPress manages action callbacks internally via the $wp_filter global array. Before version 7.1, WordPress used the spl_object_hash() PHP function to generate unique alphanumeric string keys for object-based callbacks.

In WordPress 7.1, the core code optimized hook registry lookups by switching to spl_object_id(), which returns unique integers instead of strings.

2. Strict Types vs. PHP 8.x

WP Rocket’s Cloudflare integration script (Cloudflare.php) declares strict typing at the top of the file:

declare(strict_types=1);

During initialization, the plugin loops through the registered callbacks on your posts to unregister cache-clearing hooks. It uses the PHP substr() function to match the callback's key:

if ( substr( $key, - strlen( $method ) ) !== $method ) {
    continue;
}

Because strict typing is enabled, PHP expects $key to be a string. When WordPress 7.1 passes an integer key from its new hook registry, PHP 8+ immediately throws a fatal TypeError rather than converting the type implicitly.

How to Fix the Crash (Step-by-Step)

Since the critical error locks you out of the WordPress dashboard, you must perform the fix using your hosting File Manager or FTP.

Step 1: Deactivate the Plugin (Bypass the White Screen)

1. Log into your hosting control panel (such as Hostinger hPanel or Namecheap cPanel) and open File Manager (or connect via FTP/SFTP).
2. Navigate to public_html/wp-content/plugins/.
3. Locate the wp-rocket folder.
4. Rename the folder to wp-rocket-disabled.
5. Refresh your website. The error will immediately clear, and your dashboard access will be restored.

Step 2: Apply the Code Guard Hotfix

If you cannot update the plugin immediately (e.g., if you are running an older stable version like 3.18.x or your license is currently inactive), you can patch the code manually in less than a minute.

1. Open /wp-content/plugins/wp-rocket-disabled/inc/ThirdParty/Plugins/CDN/Cloudflare.php in your File Manager editor.
2. Search for the unregister_callback method (typically located near the bottom of the file).
3. Find the foreach loop that checks the $key:

foreach ( $original_wp_filter[ $priority ] as $key => $config ) {

    if ( substr( $key, - strlen( $method ) ) !== $method ) {
        continue;
    }

    unset( $wp_filter[ $hook ]->callbacks[ $priority ][ $key ] );
}

4. Insert the is_string($key) check right after the foreach statement to skip numeric hook IDs:

foreach ( $original_wp_filter[ $priority ] as $key => $config ) {

    // Add this type-check guard for WordPress 7.1 compatibility:
    if ( ! is_string( $key ) ) {
        continue;
    }

    if ( substr( $key, - strlen( $method ) ) !== $method ) {
        continue;
    }

    unset( $wp_filter[ $hook ]->callbacks[ $priority ][ $key ] );
}

5. Save the file.
6. Rename the folder back to wp-rocket and click Activate in the WordPress Plugins page.

Tradeoffs and Operational Impact

Naveen Gaur is a WordPress Performance Specialist & Full-Stack Consultant helping business owners and agencies build, fix, and maintain high-performing web platforms.