Debugging PHP Errors with WP_DEBUG

Debugging PHP Errors with WP_DEBUG: A Comprehensive Guide

Introduction
Debugging PHP errors in a WordPress environment can be challenging without the right tools. Fortunately, WordPress comes with a built-in debugging mechanism powered by the WP_DEBUG constant. Whether you’re a theme or plugin developer—or a site administrator hunting down a fatal error—this guide will walk you through every facet of WP_DEBUG and its related constants, best practices for development vs. production, and advanced techniques for isolating elusive issues.

Why WP_DEBUG Matters

By default, WordPress suppresses non-fatal PHP notices, warnings, and deprecated function messages. While this creates a clean user experience, it can hide critical clues during development. Enabling WP_DEBUG forces WordPress and PHP to display or log these messages, giving you visibility into:

• Syntax errors: Unclosed braces, commas, unexpected tokens.
• Deprecated functions: Warnings about functions removed or slated for removal.
• Undefined variables: Notices that reveal potential logic bugs.
• Database query logs: Identifying slow or erroneous SQL queries (with SAVEQUERIES).

1. Enabling WP_DEBUG

To turn on WordPress debugging, edit your wp-config.php file, typically located in your site’s root directory. Locate the line that defines WP_DEBUG (or add it if missing) and set it to true:

define(WP_DEBUG, true)

Tip: Place this line above the comment / Thats all, stop editing! Happy blogging. / to ensure WordPress picks it up early in the loading process.

2. Core Debugging Constants

While WP_DEBUG is the master switch, WordPress offers several companion constants:

Constant	Purpose	Default
WP_DEBUG_DISPLAY	Whether to display errors and warnings in the HTML output.	true
WP_DEBUG_LOG	Whether to log errors to wp-content/debug.log.	false
SCRIPT_DEBUG	Loads non-minified core CSS and JS files for more readable stacks.	false
SAVEQUERIES	Saves database queries to an array for analysis.	false

Example of a robust debug configuration:

define(WP_DEBUG, true)
define(WP_DEBUG_LOG, true)
define(WP_DEBUG_DISPLAY, false)
define(SCRIPT_DEBUG, true)
define(SAVEQUERIES, true)

3. Display vs. Logging

WP_DEBUG_DISPLAY

When WP_DEBUG_DISPLAY is true, PHP errors are output directly to the browser—valuable for local development but a security risk on public sites.

WP_DEBUG_LOG

With WP_DEBUG_LOG enabled, all errors (including notices and warnings) are appended to wp-content/debug.log. Inspect this file in real time:

tail -f /path/to/wordpress/wp-content/debug.log

4. Advanced Debugging Techniques

1. Debugging AJAX Calls
   Use browser dev tools and look at network responses—errors often appear in the JSON payload. Combine with error_log() statements in your PHP callbacks.
2. Profiling with SAVEQUERIES
   After enabling SAVEQUERIES, display query information:

   queries = wpdb->queries
   echo ltpregt . print_r( queries, true ) . lt/pregt

   Identify slow queries by checking the duration column.

3. Xdebug Integration
   Install Xdebug (xdebug.org) on your local server, configure breakpoints in your IDE (e.g., PhpStorm). Step through code to inspect variables and call stacks.

5. Best Practices

• Never leave WP_DEBUG_DISPLAY enabled on a production site.
• Regularly audit debug.log for recurring notices and warnings.
• Remove debug statements and constants before deploying to production to optimize performance.
• Use version control (Git) to track changes in wp-config.php and related custom code.
• Consider using dedicated debugging plugins like Query Monitor for an enhanced UI within the WordPress admin bar.

6. Common Error Scenarios

Here are a few examples of typical PHP errors you might encounter and how WP_DEBUG helps:

• Parse error, unexpected ‘lt’: Indicates broken PHP syntax—often a missing semicolon or bracket.
• Call to undefined function: The plugin or theme is calling a function not loaded in the current context.
• Deprecated: Shows legacy functions—update to current WordPress APIs to ensure future compatibility.

Conclusion

Enabling and mastering WP_DEBUG transforms error hunting from a guessing game into a systematic process. Whether you display errors in real time, log them for later review, or integrate professional profiling tools, comprehensive debugging is an essential part of any WordPress development workflow. By following the guidelines outlined above, you can squash bugs faster, write cleaner code, and deliver more reliable websites.