Snipped of settings.php that shows the environment specific include files

settings.php

<?php
// SNIP REST OF settings.php ABOVE

if(defined('PANTHEON_ENVIRONMENT')) {
  switch(PANTHEON_ENVIRONMENT) {
    case 'live':
      $env_settings = dirname(__FILE__) . '/settings.live.php';
      break;
    case 'test':
      $env_settings = dirname(__FILE__) . '/settings.test.php';
      break;
    case 'dev':
      $env_settings = dirname(__FILE__) . '/settings.dev.php';
      break;
    default:
      //This will be multidev instances - use dev settings
      $env_settings = dirname(__FILE__) . '/settings.dev.php';
      break;
  }

  if (file_exists($env_settings)) {
    include $env_settings;
  }
}