WordPress debug mode enables us to see the errors when we develop. It is a necessary step when building something new to be sure that our code works well.

To turn on the debug mode, we have to set the WP_DEBUG constant in the wp-config.php to true:

// Enable WP_DEBUG mode
define('WP_DEBUG', true);

We can also set up (also in the wp-config.php file) where we want to see the errors and warnings, in a log file or on the site:

// Debug logging into a separate file (/wp-content/debug.log)
define('WP_DEBUG_LOG', true);

// Disable error and warning display on site
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors', 0);

We can also load the development versions of the core JS and CSS files to simplify debugging:

// Load the development versions of core JS and CSS files
define('SCRIPT_DEBUG', true);

The fully enable a manageable WordPress debug setup use the following code:

// Enable WP_DEBUG mode with logging
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors', 0);
define('SCRIPT_DEBUG', true);

Note that you can also use it on a live site because the messages are only shown in a log file.