WP-DEBUG
WP_DEBUG is a PHP constant (a permanent global variable) that can be used to trigger the "debug" mode throughout WordPress. It is assumed to be false by default and is usually set to true in the wp-config.php file on development copies of WordPress.
define('WP_DEBUG', true); define('WP_DEBUG', false);
WP_DEBUG_LOG
WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).
Note that this allows you to write to /wp-content/debug.log using PHP's built in error_log() function, which can be useful for instance when debugging AJAX events.
