WordPress Debugging

Sooner or later something will break on your site: a white page, a plugin error, or the message There has been a critical error on this website. WordPress has a built in debug mode which shows what is going wrong.

The debug constants

Add these lines in wp-config.php, above the line “That’s all, stop editing”:

define( 'WP_DEBUG', true );          // turn on debug mode
define( 'WP_DEBUG_LOG', true );      // save errors in wp-content/debug.log
define( 'WP_DEBUG_DISPLAY', false ); // do not show errors on the page

Constant	Purpose
WP_DEBUG	Main switch of the debug mode
WP_DEBUG_LOG	Writes the errors to wp-content/debug.log
WP_DEBUG_DISPLAY	Shows the errors on the page. Keep it false on a live site
SCRIPT_DEBUG	Loads the not minified CSS and JS of WordPress
SAVEQUERIES	Saves all database queries, so tools can show them

A real example

We create a small plugin with a mistake: it uses a variable which does not exist.

<?php
/**
 * Plugin Name: Buggy Demo
 */
add_action( 'wp_footer', function () {
	echo $total_price;              // this variable was never created
	error_log( 'Footer code ran' ); // our own message in the log
} );

We removed the time at the start of each log line to keep the output short. The page still loads (code 200), but the problem is written in the log with the file name and the line number. Now we clean up the demo, because the debug mode must not stay on:

Write your own log messages

Use error_log() in your code. For arrays and objects use print_r with the second parameter true:

error_log( 'Order saved: ' . $order_id );
error_log( print_r( $data, true ) );

Fix the critical error

• Open wp-content/debug.log (or the error log of your host) to see the file and the line.
• If a plugin is the reason, rename its folder with FTP or the file manager (for example plugins/bad-plugin to plugins/bad-plugin-off). WordPress will deactivate it.
• If the theme is the reason, rename the theme folder. WordPress will fall back to a default theme.
• Check your PHP version. A new PHP version can break old code.
• Use the recovery mode link which WordPress sends to the admin email.

Helpful tools

• Query Monitor plugin: shows queries, hooks, errors and slow code.
• Site Health (Tools > Site Health): checks versions, modules and common problems.
• Your browser developer tools for JavaScript errors.

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