thinkphp5.1 获取根目录

use think\facade\Env;

Env::get('root_path'); 

其他路径获取：

// /thinkphp/library/think/App.php    
// 设置路径环境变量
$this->env->set([
    'think_path'   => $this->thinkPath,
    'root_path'    => $this->rootPath,
    'app_path'     => $this->appPath,
    'config_path'  => $this->configPath,
    'route_path'   => $this->routePath,
    'runtime_path' => $this->runtimePath,
    'extend_path'  => $this->rootPath . 'extend' . DIRECTORY_SEPARATOR,
    'vendor_path'  => $this->rootPath . 'vendor' . DIRECTORY_SEPARATOR,
]);