* One of NULL, 'hostspec', 'port', 'username', 'password', 'database'.
* @return string|array|NULL
* If $part is omitted, return full DSN array.
* One of NULL, 'hostspec', 'port', 'username', 'password', 'database'.
* @return string|array|NULL
* If $part is omitted, return full DSN array.