Enable 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.
For information on other debugging tools built into WordPress see Debugging in WordPress
define( 'WP_DEBUG', true );
define( 'WP_DEBUG', false );