如何将另一个文件中的变量赋给类常量?

const KEY_USER_NAME = 'user_name';
const KEY_USER_AGE = 'user_age';
// ...

require $path_to_somewhere . '/keys.php';

class YourClass 
{
  const API_USER_NAME = KEY_USER_NAME;
  const API_USER_AGE = KEY_USER_AGE;

  // ...
}

https://github.com/vlucas/phpdotenv

as the documentation states

const

const FOO = 'BAR';

require_once('constants.php');
var_dump(defined('FOO'));
// outputs bool(true)

define()

$bar = 'baz';
define('FOO', $bar);

echo FOO;
// outputs 'baz';