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How to Fix the “Notice: ob_end_flush(): Failed to send buffer of zlib output compression” Error in WordPress

ByMuhammad Imanudin Posted on

If you’ve encountered the error “Notice: ob_end_flush(): Failed to send buffer of zlib output compression” in WordPress, don’t worry! This is a common issue, often caused by conflicts between WordPress’s output buffering and server settings, especially when Zlib Compression is enabled. In this article, we’ll explore multiple ways to fix this error, including creating a custom plugin and using Must-Use Plugins (mu-plugins) for a permanent solution.

Method 1: Hide the Error by Modifying WP Debug

If you want to hide the error without diving deep into debugging, you can modify the wp-config.php file. Here’s how:

  1. Open the wp-config.php file in the root folder of your WordPress installation.
  2. Locate the line with define('WP_DEBUG', false);.
  3. Replace or add the following code:
// Enable WP_DEBUG mode
define('WP_DEBUG', true);

// Enable Debug logging to the /wp-content/debug.log file
define('WP_DEBUG_LOG', true);

// Disable display of errors and warnings
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors', 0);

// Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
define('SCRIPT_DEBUG', true);

This will prevent the error from being displayed on the screen, but it will still be logged in the debug.log file for further investigation.

Method 2: Proper Fix by Modifying ob_end_flush

For a more robust solution, you can replace the default WordPress wp_ob_end_flush_all() function with a custom one. Here’s how:

  1. Open the functions.php file in your theme (via Appearance > Theme Editor or directly editing the file).
  2. Add the following code:
/**
 * Proper ob_end_flush() for all levels
 *
 * This replaces the WordPress `wp_ob_end_flush_all()` function
 * with a replacement that doesn't cause PHP notices.
 */
remove_action('shutdown', 'wp_ob_end_flush_all', 1);
add_action('shutdown', function() {
   while (@ob_end_flush());
});

This code removes the default WordPress function and replaces it with a safer alternative.

Method 3: Disable Zlib Output Compression

If the above methods don’t work, you can try disabling Zlib Output Compression. Here’s how:

a. Via .htaccess

  1. Open the .htaccess file in the root folder of your WordPress installation.
  2. Add the following line:
php_flag zlib.output_compression off

b. Via wp-config.php

  1. Open the wp-config.php file.
  2. Add the following line:
@ini_set('zlib.output_compression', 'Off');

Method 4: Create a Custom Plugin

If you want a more flexible and reusable solution, you can create a custom plugin. Here’s how:

  1. Go to the wp-content/plugins/ directory.
  2. Create a new folder named fix-ob-end-flush-error.
  3. Inside the folder, create a file named fix-ob-end-flush-error.php.
  4. Add the following code to the file:
<?php
/*
Plugin Name: Fix ob_end_flush() Error
Plugin URI: https://example.com/
Description: A simple plugin to fix the "Notice: ob_end_flush(): Failed to send buffer of zlib output compression" error in WordPress.
Version: 1.0
Author: Your Name
Author URI: https://example.com/
License: GPLv2 or later
Text Domain: fix-ob-end-flush-error
*/

// Prevent direct access to the file
if (!defined('ABSPATH')) {
    exit;
}

/**
 * Proper ob_end_flush() for all levels
 *
 * This replaces the WordPress `wp_ob_end_flush_all()` function
 * with a replacement that doesn't cause PHP notices.
 */
function fix_ob_end_flush_error() {
    remove_action('shutdown', 'wp_ob_end_flush_all', 1);
    add_action('shutdown', function() {
        while (@ob_end_flush());
    });
}
add_action('init', 'fix_ob_end_flush_error');
  1. Go to your WordPress dashboard, navigate to Plugins > Installed Plugins, and activate the Fix ob_end_flush() Error plugin.

Method 5: Use Must-Use Plugins (mu-plugins)

For a permanent solution that always runs regardless of the active theme or plugins, you can use Must-Use Plugins (mu-plugins). Here’s how:

  1. Go to the wp-content/ directory.
  2. If it doesn’t already exist, create a folder named mu-plugins.
  3. Inside the mu-plugins folder, create a file named fix-ob-end-flush-error.php.
  4. Add the following code to the file:
<?php
/*
Plugin Name: Fix ob_end_flush() Error (Must-Use)
Description: A must-use plugin to fix the "Notice: ob_end_flush(): Failed to send buffer of zlib output compression" error in WordPress.
Version: 1.0
Author: Your Name
*/

// Prevent direct access to the file
if (!defined('ABSPATH')) {
    exit;
}

/**
 * Proper ob_end_flush() for all levels
 *
 * This replaces the WordPress `wp_ob_end_flush_all()` function
 * with a replacement that doesn't cause PHP notices.
 */
function fix_ob_end_flush_error() {
    remove_action('shutdown', 'wp_ob_end_flush_all', 1);
    add_action('shutdown', function() {
        while (@ob_end_flush());
    });
}
add_action('init', 'fix_ob_end_flush_error');
  1. Save the file. WordPress will automatically load and run this plugin because it’s in the mu-plugins folder.

Why Does This Error Occur?

This error typically happens due to conflicts between WordPress’s output buffering and server settings, especially when Zlib Compression is enabled. The methods above provide solutions to either hide the error, fix the underlying issue, or disable the conflicting feature.

By following these methods, you can effectively resolve the “Notice: ob_end_flush(): Failed to send buffer of zlib output compression” error in WordPress. Whether you choose to hide the error, fix it properly, or create a custom plugin, these solutions will help you maintain a smooth-running website. Happy troubleshooting! 🚀

Additional Tips

Always back up your website before making changes to files.

If you’re using a caching plugin, try deactivating it temporarily to see if the error persists.

Keep WordPress, themes, and plugins updated to avoid compatibility issues.