WP_DEBUG via query parameter, WP_DEBUG dev stack

debug.php

// Allows you to add ?debug=true query parameter
// For live sites in which a dev environment is not possible.

if ( isset($_GET['debug']) && $_GET['debug'] == 'true') {
define('WP_DEBUG', true);
}

// A nice stack recommended by @jjeaton,
// for dev environments:

// Debug Settings
define('WP_DEBUG', true);
if (WP_DEBUG) {
  define('WP_DEBUG_LOG', true);
  define('WP_DEBUG_DISPLAY', false);
  define('SAVEQUERIES', true);
  define('SCRIPT_DEBUG', true);
  @ini_set('display_errors',0);
  if ( function_exists( 'xdebug_disable' ) ) {
    xdebug_disable();
  }
}