Enabling WP_DEBUG will cause all PHP errors, notices and warnings to be displayed. This is likely to modify the default behavior of PHP which only displays fatal errors and/or shows a white screen of death when errors are reached.
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 );