今天看到的一段PHP代码，好几遍才懂

public function get_config($s = null)
{
if (empty($s))
return $this->config;

$v = explode('.', $s);
$config = $this->config;
foreach ($v as $u) {
if (!is_array($config)
|| !array_key_exists($u, $config))
return null;
$config = $config[$u];
}
return $config;
}