Debug WordPress

Tools and techniques for debugging WordPress.

Incoming HTTP requests

Dump HTTP request headers to debug-request-headers.log.
Install this plugin.
Follow error log: tail -f wp-content/debug-request-headers.log

Dump POST request body to the error log.
Copy this snippet to wp-config.php.
Follow error log: tail -f error_log

Tools

• For permalinks: rewrite-rules-inspector plugin
• For AJAX requests: https://github.com/szepeviktor/qm-ajax
• For REST requests: https://querymonitor.com/wordpress-debugging/rest-api-requests/

HTTP response headers

Open developer tools in your browser.

Tools

• https://redbot.org/
• https://securityheaders.com/
• https://www.webpagetest.org/

HTTP response body

View page source in your browser.

Tools

• https://validator.w3.org/
• https://www.webpagetest.org/

PHP code

Dump triggered hooks (actions and filters) to a file.
Copy this MU plugin to wp-content/mu-plugins/.
Follow the log: tail -f wp-content/debug-hooks.log

Alternatively enable debugging in wp-config.

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
ini_set('display_errors', '0');

Follow the log: tail -f wp-content/debug.log

What is happening?

1. Hooks: log in all action.
2. SQL queries: log at the end of wpdb::_do_query() in wp-includes/class-wpdb.php
3. Object cache: log in WP_Object_Cache::get() and WP_Object_Cache::set() in wp-content/object-cache.php

Tools

• Plugin Identifier Search https://plugintests.com/search-ids

PHP errors

Follow PHP error log: tail -f error_log

Outbound HTTP requests

Log requests and responses.

Tools

• snitch plugin
• log-http-requests plugin

Cron events

Cron can be web-based and CLI-based.

Log events and failures.

Background queue jobs

Check Action Scheduler logs.

Database queries

Log queries before they are executed, see EXPLAIN statement.
Copy this snippet to wp-includes/class-wpdb.php.
Follow the log: tail -f wp-content/debug-queries.log

Log failed queries.

Tools

• index-wp-mysql-for-speed plugin
• Database Performance tuning

Emails

Log sent emails and sending failures.

Support my work

Please consider supporting my work as these lists take years to compile.

Thank you!

General debug tools

• query-monitor plugin
• Test Mode plugin
• Troubleshooting Mode of health-check plugin
• airplane-mode plugin
• whats-running plugin
• frontend-debugger plugin
• https://themecheck.info/