if (! defined('ENV')) {
    $production  = 'example.com';
    $staging     = 'test.example.com';
    $local       = 'example.dev';

    switch (strtolower($_SERVER['HTTP_HOST'])) {
        case $production:
            define('ENV', 'production');
            define('ENV_FULL', 'Production');
            define('ENV_DEBUG', false);
            define('ENV_DOMAIN', $production);
        break;

        case $staging:
             ...
        break;

        case $local:
             ...
        break;
    }
}

Use of undefined constant ENV - assumed 'ENV'
Fatal error: require(): Failed opening required '/var/www/html/config/environments/config.ENV.php'