How can I resolve the php notice “Constant EMPTY_TRASH_DAYS already defined”

When defining any WordPress constants in wp-config.php you need to do it before this line:

require_once( ABSPATH . 'wp-settings.php' );

That line loads many of WordPress’s default constants, and if you haven’t already defined them yourself then they’ll be defined in that line, meaning that any of them that you try to define after this line will have already been defined.

error code: 523