Notice: ob_end_flush(): Failed to send buffer of zlib output compression (0)

The notice “ob_end_flush(): Failed to send buffer of zlib output compression (0)” in PHP indicates an issue with output buffering and zlib compression. This often occurs in environments where zlib.output_compression is enabled in php.ini, and there’s a conflict or problem when PHP attempts to flush the compressed output buffer.

Common Causes and Solutions:

• WordPress-Specific Issues:
  • wp_ob_end_flush_all() function: In WordPress, the wp_ob_end_flush_all() function, hooked to the shutdown action, can sometimes conflict with zlib.output_compression.
  • Solution: A common fix involves replacing the default WordPress shutdown action with a custom one that handles output buffering more robustly. This can be done by adding code to a custom plugin or a mu-plugin file (e.g., wp-content/mu-plugins/fix-zlib.php):

Code

        <?php
        /**
         * Fix zlib Error
         */
        remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );
        add_action( 'shutdown', static function () {
            $levels = ob_get_level();
            for ( $i = 0; $i < $levels; $i ++ ) {
                @ob_end_flush();
            }
        }, 9999999 );
        ?>

• Server Configuration (php.ini):
  • zlib.output_compression: If this setting is enabled and causing issues, it might be necessary to disable it.
  • Solution: Modify your php.ini file to set zlib.output_compression = Off. Remember to restart your web server after making this change.
• Missing Closing Tags or Output Conflicts:
  • In some cases, especially in templating engines like Blade (Laravel), missing closing statements or unexpected output before the end of the script can lead to this notice.
  • Solution: Review your code, particularly template files, for any unclosed HTML tags or accidental output before the intended content.
• Plugin/Theme Conflicts (WordPress):
  • Certain plugins or themes might interfere with output buffering or zlib compression, leading to this notice.
  • Solution: Deactivate plugins one by one to identify the culprit. If a specific plugin or theme is causing the issue, consider looking for updates or alternative solutions.