W3TC should check if WP_CACHE is already defined before adding PHP code defining it.

[ckeeney]

I am using roots.io Bedrock, which refactors the WP config and folder structure quite a bit. W3TC adds code to the top of wp-config.php, and that is not where my configuration is stored:

<?php
/** Enable W3 Total Cache */
define('WP_CACHE', true); // Added by W3 Total Cache

/**
 * Do not edit this file. Edit the config files found in the config/ dir instead.
 * This file is required in the root directory so WordPress can find it.
 * WP is hardcoded to look in its own directory or one directory up for wp-config.php.
 */
require_once dirname(__DIR__) . '/vendor/autoload.php';
require_once dirname(__DIR__) . '/config/application.php';
require_once ABSPATH . 'wp-settings.php';

I moved the constant to application.php, but W3TC re-adds the code to wp-config.php without realizing the constant is already defined. This results in a PHP error for redefining a constant.

PHP Fatal error:  Uncaught Roots\\WPConfig\\Exceptions\\ConstantAlreadyDefinedException: Aborted trying to redefine constant 'WP_CACHE'. `define('WP_CACHE', ...)` has already been occurred elsewhere. 

[jamieburchell]

See #752